Bump required GitPython (#68)

Fix #65. 

GitPython 2.18 is now out with the required fix. 

gitpython-developers/GitPython#687 (comment)

Author: hugovk

requirements.txt

@@ -1,4 +1,4 @@
-GitPython>=2.1.1
+GitPython>=2.1.8
 colorama>=0.3.7
 termcolor>=1.1.0
 click>=6.0.0

setup.py

@@ -5,7 +5,7 @@
     name="git-up",
     version="1.4.5",
     packages=find_packages(),
-    install_requires=['GitPython>=2.1.1', 'colorama>=0.3.7',
+    install_requires=['GitPython>=2.1.8', 'colorama>=0.3.7',
                       'termcolor>=1.1.0', 'click>=6.0.0',
                       'six>=1.10.0'],