Require gitdb (at least version 4)

#59

Author: Byron

requirements.txt

@@ -1 +1 @@
-smmap>=3
+gitdb>=4

setup.py

@@ -20,7 +20,7 @@
     packages=('gitdb', 'gitdb.db', 'gitdb.utils', 'gitdb.test'),
     license="BSD License",
     zip_safe=False,
-    install_requires=['smmap>=3'],
+    install_requires=['gitdb>=4'],
     long_description="""GitDB is a pure-Python git object database""",
     python_requires='>=3.4',
     # See https://pypi.python.org/pypi?%3Aaction=list_classifiers