Enum support for RailsAdmin, Tests are running now, corrected Dependencies

Hi, as i wrote on the pull request https://github.com/nofxx/symbolize/pull/33:

* added the enum support for RailsAdmin with the code from @rewritten (:+1:)
* the tests are now able to run with 'bundle exec rspec'. i wonder that the tests were able to run previously!
* the dependencies were completely old set! for example mongoid has to be at least 3.0 but the dependency was set to 2.3!
 * because there was a Gemfile.lock (that i removed for good reason) i thing that you the developer just updated your local gems but never the dependencies.

please accept this pull request and close https://github.com/nofxx/symbolize/pull/33.

btw: this path was done in 3 hours, please dont make it waste of time. thanks