Add rubocop linter to travis

[MaicolBen]

Rubocop issues were fixed in #1125 but it wasn't added to travis, so we didn't fix the last issues with the new code changes.

[dks17]

This PR is useful for maintaining the repository. Try follow these steps:

1. rebase
2. delete .rubocop.yml and .rubocop_todo.yml
3. run rubocop --auto-gen-conf
4. add bundle exec rubocop below of this line
   

   devise_token_auth/.travis.yml

   Line 59 in 0727ac7

   - bundle install --jobs=3 --retry=3 --gemfile $GEMFILE_AR

And add rubocop step to Contribution Guidelines like: bundle exec rubocop -a and bundle exec rubocop --auto-gen-conf after change is made.