BF: install_requires not just requires for setup() call

Originally detected while running DataLad tests on CRON
  datalad/datalad#3395

Author: yarikoptic

setup.py

@@ -80,7 +80,7 @@ def _stamp_version(filename):
     package_dir={'git': 'git'},
     license="BSD License",
     python_requires='>=2.7, !=3.0.*, !=3.1.*, !=3.2.*, !=3.3.*',
-    requires=requirements,
+    install_requires=requirements,
     tests_require=requirements + test_requirements,
     zip_safe=False,
     long_description="""GitPython is a python library used to interact with Git repositories""",