Update GitPython dependency

Fixes gitdb import issue
See gitpython-developers/GitPython#983

Author: danielmcmir

Diff for: setup.py

@@ -15,10 +15,10 @@
     long_description=long_description,
     long_description_content_type="text/markdown",
     install_requires=[
-        'GitPython == 2.1.11'
+        'GitPython == 2.1.15'
    ],
    classifiers=[
         "Programming Language :: Python :: 3",
         "License :: OSI Approved :: MIT License"
     ]
-)
+)