Capistrano v3 implementation for bugnsag:deploy (that works!)

The implementation in #64 (https://github.com/bugsnag/bugsnag-ruby/blob/master/lib/bugsnag/tasks/bugsnag.cap) seems to be a dud. There's no way `bugsnag:deploy` ever gets loaded and the commands are not Capistrano v3 compatible.

Here's a basic working implementation using the new v3 defaults and conventions.