Merge pull request #8692 from edx/derf/pymongo_2.8

upgrade to pymongo 2.8 to support updated authenticated method SCRAM-SHA-1 for mongodb-server 3.0

requirements/edx/base.txt

@@ -61,7 +61,7 @@ pycrypto>=2.6
 pygments==2.0.1
 pygraphviz==1.1
 PyJWT==1.0.1
-pymongo==2.7.2
+pymongo==2.8.1
 pyparsing==2.0.1
 python-memcached==1.48
 python-openid==2.2.5