Bundler integrated Capistrano provider

Executes bundle install during deployment without surrounding bundler
environment. After that, cap deploy is run normally.
Install only gems from heaven and deployment groups
Merge branch 'bundler-capistrano' into flowdock

* bundler-capistrano:
  Install only gems from heaven and deployment groups
  Bundler integrated Capistrano provider

Conflicts:
	lib/heaven/provider/bundler_capistrano.rb
Read ssh key to environment variable