Rails 4 bump + bug fix

2 things in here:

- Bumping rails to latest 4.0.4.rc1.
- When upgrading rails I found a issue:
  https://github.com/rails/rails/commit/a4d4afd2b4e5d0c10fc23b24260f8bc1c834e9a6 this was fixed on rails, which means that empty strings wont be saved in a array. We were sending a empty `favorite_languages` in every post, which IMO is not correct. Fixed that too.


review @schneems

cc @rafaelfranca @carlosantoniodasilva