Capistrano: deploy from fixed branch

Not the 'current' branch in the local clone fo the repo.

I feel like deploying from the current branch in the local clone is a
bit presumptous. I expect most people want to deploy from one branch
while allowing for development of several feature branches.

Right now you always have to remember to check out master before
deploying or you may deploy your feature branch (which may not even be
pushed).