Handling unsupported format requests with error page

[vitorbaraujo]

• Rescuing ActionView::MissingTemplate with not_found method located in app/controllers/application_controller.rb
• Modifying not_found method to render page telling that format given is unsupported
• Updating related tests

This PR closes #301

[rafamanzo]

Nice modifications!

I have a few good practice advises for your future contributions:

• It has been some time that we've adopted some rules for commit messages (https://chris.beams.io/posts/git-commit/#seven-rules) that have been really helpful for us on understanding modifications both for revision and bugfix purposes
• I see you have fixed more than just Handle unsupported format requests #301 by modifying the Gemfile. That's a nice fix, but have in mind to make it a separate PR in the future. This makes review easier and allow more code to reach the master faster

Welcome and thanks for the nice work!