Pin transitive dep on gitdb to avoid ModuleNotFoundError

gitpython-developers/GitPython#983

======================================================================
ERROR: test_doctests (unittest.loader._FailedTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/builds/azul/azul/test/test_doctests.py", line 50, in load_tests
    tests.addTests(doctest.DocTestSuite(load_module(root + '/scripts/check_branch.py', 'check_branch')))
  File "/builds/azul/azul/src/azul/modules.py", line 19, in load_module
    spec.loader.exec_module(module)
  File "<frozen importlib._bootstrap_external>", line 678, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/builds/azul/azul/scripts/check_branch.py", line 4, in <module>
    import git
  File "/build/.venv/lib/python3.6/site-packages/git/__init__.py", line 38, in <module>
    from git.exc import *                       # @nomove @IgnorePep8
  File "/build/.venv/lib/python3.6/site-packages/git/exc.py", line 9, in <module>
    from git.compat import UnicodeMixin, safe_decode, string_types
  File "/build/.venv/lib/python3.6/site-packages/git/compat.py", line 16, in <module>
    from gitdb.utils.compat import (
ModuleNotFoundError: No module named 'gitdb.utils.compat'
----------------------------------------------------------------------

requirements.dev.txt

@@ -4,6 +4,7 @@ docker==3.4.1
 locust==1.1.1
 pytest==3.2.3
 coverage==4.0.3
+gitdb2==2.0.6  # transitive, pin to avoid https://github.com/gitpython-developers/GitPython/issues/983
 gitpython==2.1.11
 google-api-python-client==1.7.8
 moto==1.3.13  # 1.3.14 does not work, possibly related to https://github.com/spulec/moto/issues/2567