freeswitch: fix cloning the stable branch

This automatically clones from the remote branch instead of
doing the branch switch locally.

The approach introduced in https://github.com/plivo/plivoframework/pull/125
does not work. It was working in the environment I was working on because
I automatically did some things to test the changes conveniently. I did *not* test
for completely clean machines. The old way would fail, because it's a shallow
clone. It only kept one version of the repo's history and *ONLY* for the master
branch. This is why it would fail when it switched to the stable branch.

I apologize for the trouble.