Moving 'six' package into gRPC install, as it's not white-listed for travis.

.travis.yml

@@ -17,8 +17,6 @@ addons:
     - python-dev
     - python-mysqldb
     - python-pip
-    # required by gRPC python files
-    - python-six
     - libssl-dev
     - g++-4.8
     - gcc-4.8

travis/install_grpc.sh

@@ -11,11 +11,14 @@ if [ "$grpc_dist" != "" ]; then
   cd $grpc_dist
 fi
 
-# for python, we'll need the latest virtualenv and tox
+# for python, we'll need the latest virtualenv and tox.
+# running gRPC requires the six package, any version will do.
 if [ "$grpc_dist" != "" ]; then
   pip install --upgrade --root $grpc_dist virtualenv tox
+  pip install --root $grpc_dist six
 else
   pip install --upgrade virtualenv tox
+  pip install six
 fi
 
 # clone the repository, setup the submodules