[AIRFLOW-3685] Move licence header check

[ffinfo]

Make sure you have checked all steps below.

Jira

• My PR addresses the following Airflow Jira issues and references them in the PR title.
  • https://issues.apache.org/jira/browse/AIRFLOW-3685

Description

Moving licence header check to a pre-test stage to prevent useless travis resources

Tests

• My PR adds the following unit tests OR does not need testing for this extremely good reason:

Commits

• 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":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Documentation

• In case of new functionality, my PR adds documentation that describes how to use it.
  • When adding new operators/hooks/sensors, the autoclass documentation generation needs to be added.
  • All the public functions and the classes in the PR contain docstrings that explain what it does

Code Quality

• Passes flake8

[kaxil]

+1 this is a good change. Approved. Waiting for the CI to pass.

[BasPH]

Wow what a coincidence... I was just about to push the PR button and see this. https://issues.apache.org/jira/browse/AIRFLOW-3686

[BasPH]

My branch is a bit more explicit: master...BasPH:bash-split-license-ci-check. The license check is a separate stage since it's not related to Flake8 IMO:

[ffinfo]

This is my version, in this case flake8 and check licence can run in parallel.
May this best one win ;)

[codecov-io]

Codecov Report

Merging #4497 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4497   +/-   ##
=======================================
  Coverage   74.75%   74.75%           
=======================================
  Files         428      428           
  Lines       29611    29611           
=======================================
  Hits        22135    22135           
  Misses       7476     7476

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