add services item mongodb to .travis.yml

Travis CI: MongoDB is not started on boot.
Merge branch 'gouf-travis_ci_with_mongodb'
fix: .travis.yml
add gem guard-rspec
update Guardfile to add rspec config
guard init rspec
Update Guardfile grouped in Rubocop and RSpec
Merge pull request #11 from gouf/guard_rspec

guard-rspec
rubocop: disable Rails/Output