[AIRFLOW-5687] Upgrade pip to 19.0.2
diff --git a/Dockerfile b/Dockerfile
index ae924be739546..bc86c9dc377a9 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -270,7 +270,7 @@ ENV PIP_NO_CACHE_DIR=${PIP_NO_CACHE_DIR}
 RUN echo "Pip no cache dir: ${PIP_NO_CACHE_DIR}"
 
 # PIP version used to install dependencies
-ARG PIP_VERSION="19.0.1"
+ARG PIP_VERSION="19.0.2"
 ENV PIP_VERSION=${PIP_VERSION}
 RUN echo "Pip version: ${PIP_VERSION}"
 
@@ -304,7 +304,7 @@ ENV AIRFLOW_CI_BUILD_EPOCH=${AIRFLOW_CI_BUILD_EPOCH}
 # And is automatically reinstalled from the scratch every month
 RUN \
     if [[ "${AIRFLOW_CONTAINER_CI_OPTIMISED_BUILD}" == "true" ]]; then \
-        pip install --no-use-pep517 \
+        pip install \
         "https://github.com/apache/airflow/archive/${AIRFLOW_BRANCH}.tar.gz#egg=apache-airflow[${AIRFLOW_EXTRAS}]" \
         && pip uninstall --yes apache-airflow; \
     fi

Make sure you have checked _all_ steps below.

### Jira

- [x] My PR addresses the following [Airflow Jira](https://issues.apache.org/jira/browse/AIRFLOW/) issues and references them in the PR title. For example, "\[AIRFLOW-XXX\] My Airflow PR"
  - https://issues.apache.org/jira/browse/AIRFLOW-5687

### Description

- [x] Here are some details about my PR, including screenshots of any UI changes:
Pendulum==1.4.4 errors

Discussion on Slack resulted this PR:
https://apache-airflow.slack.com/archives/CCPRP7943/p1571319320145100
https://apache-airflow.slack.com/archives/CCPRP7943/p1571320302147400

Failed build: https://travis-ci.com/ashb/airflow/jobs/246690984
```
Complete output from command /usr/local/bin/python /usr/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py get_requires_for_build_wheel /tmp/tmpGuP1ar:
  Traceback (most recent call last):
    File "/usr/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 207, in <module>
      main()
    File "/usr/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 197, in main
      json_out['return_val'] = hook(**hook_input['kwargs'])
    File "/usr/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 54, in get_requires_for_build_wheel
      return hook(config_settings)
    File "/tmp/pip-build-env-_krb5E/overlay/lib/python2.7/site-packages/setuptools/build_meta.py", line 115, in get_requires_for_build_wheel
      return _get_build_requires(config_settings, requirements=['wheel'])
    File "/tmp/pip-build-env-_krb5E/overlay/lib/python2.7/site-packages/setuptools/build_meta.py", line 101, in _get_build_requires
      _run_setup()
    File "/tmp/pip-build-env-_krb5E/overlay/lib/python2.7/site-packages/setuptools/build_meta.py", line 85, in _run_setup
      exec(compile(code, __file__, 'exec'), locals())
    File "setup.py", line 47, in <module>
      from build import *
    File "/usr/local/lib/python2.7/site-packages/pip/_vendor/pep517/build.py", line 7, in <module>
      from pip._vendor import pytoml
  ImportError: No module named pip._vendor
```
More details: https://github.com/pypa/pip/issues/6190

Here is the comment from the maintainer: https://github.com/pypa/pip/issues/6163#issuecomment-462014864 that says pip==19.0.2 fixes that


### Tests

- [x] My PR adds the following unit tests __OR__ does not need testing for this extremely good reason:

### Commits

- [x] My commits all reference Jira issues in their subject lines, and I have squashed multiple commits if they address the same issue. In addition, my commits follow the guidelines from "[How to write a good git commit message](http://chris.beams.io/posts/git-commit/)":
  1. Subject is separated from body by a blank line
  1. Subject is limited to 50 characters (not including Jira issue reference)
  1. Subject does not end with a period
  1. Subject uses the imperative mood ("add", not "adding")
  1. Body wraps at 72 characters
  1. Body explains "what" and "why", not "how"

### Documentation

- [x] In case of new functionality, my PR adds documentation that describes how to use it.
  - All the public functions and the classes in the PR contain docstrings that explain what it does
  - If you implement backwards incompatible changes, please leave a note in the [Updating.md](https://github.com/apache/airflow/blob/master/UPDATING.md) so we can assign it to a appropriate release
