SSHKit::Backend::Printer.invoke no longer exists.

This includes fix #107.

Fix #107 removes a method alias in `SSHKit::Backend::Printer` that caused a problem for users of capistrano. It prevented the capistrano [`invoke`](https://github.com/capistrano/capistrano/blob/master/lib/capistrano/dsl.rb#L13) method from being called from within an `on` block. The alias was seldom used so it was removed.

More info available in capistrano/capistrano#912.