Refactoring with Rubocop #1.

* `Rakefile`
* `nobrainer.gemspec`
* `lib/nobrainer.rb`
* `lib/rails/*`
* `lib/no_brainer/*.rb` (without subdirectories and their `*.rb` files in `lib/no_brainer/.`)

Most of changes are suggested by `Rubocop`. Please, let me know if you are familiar with this tool and if you agree with most of their approaches, because i would like to refactor all `nobrainer` files using this tool.

It would be awesome if you could setup [code climate](https://codeclimate.com/github/signup) (AFAIR it is free for open source projects) and [travis-ci](travis-ci.org).