Use domain + year in the ruby12 deploy config

This style allows us to avoid changes in the deploy config once the
course is superseded by a new one.
Remove a duplicated bundle install during deployment

It is handled by the default Capistrano Bundler tasks included in
Capfile by bundler/capistrano.
Don't define "spec" tasks in production

The reason is we don't have the rspec gem loaded in production or
during deploy. See more in the discussion in commit #9809ab23.

This is not a good solution, but I need to get the code deployed fast.
Suggestions for better ways are welcome.