Require at least cap 3.1.0, to simplify our code.

This will mean less repetition and workarounds now that we don't need
to support 3.0.
Remove cap 3.0 deploy:restart workaround
Remove symlink workaround (not needed in cap 3.1)
Remove our custom deploy:migrate implementation.

Better to just use the one provided by capistrano/rails.
Clarify we need master branch of capistrano
Honor no_release flag for all ruby/rails-related tasks