Upgrade pip to 7.1.2

re-opening an old PR. See discussion at https://github.com/edx/edx-platform/pull/9356

requirements/edx/pre.txt

@@ -13,7 +13,7 @@ setuptools==18.0.1
 numpy==1.6.2
 
 # Needed to make sure that options in base.txt are allowed
-pip==6.0.8
+pip==7.1.2
 
 # Needed for meliae
 Cython==0.21.2