Include certain helpers only for controller specs

Travis CI was failing because of the conflict between the `sign_out` method
meant for integration tests and the `sign_out` method used for controller specs.

Example failure: https://travis-ci.org/thoughtbot/clearance/jobs/4730477
Bump up gemfile versions in travis-ci config
Fix broken links in README

* GitHub changed how links work in Markdown files:
  https://github.com/blog/1395-relative-links-in-markup-files
* Link to i18n translations in README so it is clear what the key
  hierarchy is that can be overriden.
* Break long lines after 80 characters.
* Order lists alphabetically.
Rename i18n keys for password email

* Take off _paragraph prefix because text does not contian <p> tags
* Change password text copy to be more clear.
* Sort i18n keys alphabetically.
* Resolves #248