Cap3 support for bundler_capistrano

Adds BRANCH env to bundler_capistrano deploy command.

Capistrano 2 can read the branch information from the command line with the -s switch, but in cap 3 this is no longer the case since the command line overrides are not visible in the global scope and thus heaven always deploys master. Convention in cap3 seems to be to pass arguments using environment variables (http://building.wanelo.com/2014/03/31/capistrano-you-have-changed.html, "Command Line Argument Overrides Have Changed"). By adding the env we can support both versions simultaneously.