Updates to Gemfile

This does:

* use micro_migrations from a regular gem (the gist sometimes crapped
  out)
* remove data_migrations (not used anyway)
* update safe_yaml, because 0.9.4 has been yanked meanwhile
* set jruby-openssl to require: false and require it manually in
  hub.rb, because this caused tons of warnings about existing constants
  when running rake