Add --upgrade pip

Needed before `pip instal --upgrade setuptools` for CentOS 7 and also doesn't hurt for any other distro.

README.rst

@@ -146,6 +146,7 @@ To install the synapse homeserver run::
 
     virtualenv -p python2.7 ~/.synapse
     source ~/.synapse/bin/activate
+    pip install --upgrade pip
     pip install --upgrade setuptools
     pip install https://github.com/matrix-org/synapse/tarball/master