Update install instructions for release candidates.

Author: behackett

doc/installation.rst

@@ -199,9 +199,9 @@ but can be found on the
 `github tags page <https://github.com/mongodb/mongo-python-driver/tags>`_.
 They can be installed by passing the full URL for the tag to pip::
 
- $ pip install https://github.com/mongodb/mongo-python-driver/tarball/2.2rc1
+ $ pip install https://github.com/mongodb/mongo-python-driver/archive/2.7rc0.tar.gz
 
 or easy_install::
 
- $ easy_install https://github.com/mongodb/mongo-python-driver/tarball/2.2rc1
+ $ easy_install https://github.com/mongodb/mongo-python-driver/archive/2.7rc0.tar.gz