Explicitly define gemfiles for Travis

Closes #122
Update readme
Add correct link to `CONTRIBUTING.md`

* Plain text rendered as `[CONTRIBUTING.md]().` which lead to
  https://github.com/thoughtbot/factory_girl_rails/blob/master (404)
Do not generate new file if `factories.rb` exists

* Instead, insert factory into `factories.rb`
* Reference: https://github.com/thoughtbot/suspenders/pull/339
* Add info on change to README