Gems maintenance

This pull request contains some upgrade of gems versions, new gems and some configuration.
Also, it removes some gems that I think was not essential( you may disagree with me, so we can negociate that :smile: ) for the project.

Summary:

* rails 3.2.14
* rspec-rails 2.14.0
* rmagick gem removed
* mini_magick gem added(in place of rmagick)
* pry removed
* better_errors ~~removed~~ *updated*
* binding_of_caller removed(this gem was not working in my installation. I think it's related to ruby 2.0)
* simplecov added
* letter_opener added

*UPDATE*:
* ruby version updated to 2.0.0