Remove unused gems; don't load assets group in CI.
Remove explicit load of diff-lcs.

It's a dependency of rspec-expectations.
Use unicorn instead of Thin webserver

This is in the name of eliminating unnecessary differences between
development and production. You must invoke unicorn with `unicorn`
rather than `rails s`. By default it listens on port 8080.
Remove watchr continuous-testing gem.

Nobody was using it :-(
Document what CanCan does in Gemfile.