Install a `salt-shaker` binary form the setup.py

We still need 'master' verion of pygit2. This will give us both of what we want

`pip install git+http://github.com/libgit/pygit2.git git+http://github.com/ministryofjustice/salt-shaker.git`

and `brew intall libgit2 --with-libssh2` on OSX will get a 'useful' libgit2 with ssh support