Remove from gemspec development deps not required to run tests

per discussion in https://github.com/troessner/reek/pull/226

The gems moved into the Gemfile group :local_development may be
used while developing the gem, but are not required for running tests.

Now, one can run the tests without installing all the development gems with
  bundle --without local_development
  bundle exec rake

The choice of 'local_development' has to do with how Bundler brings
evaulates the gemspec development dependencies as both development
and test dependencies.
Faster Travis Builds with Caching

See http://about.travis-ci.org/docs/user/caching/

Use travis gem CLI to clear https://github.com/travis-ci/travis#cache

API ref https://api.travis-ci.com/docs/#/repos/:owner_name/:name/caches

h/t @moonglum
https://github.com/hcking/hcking/blob/dd0c778a64c6478cdad89d14cda033551759312f/.travis.yml
Make explicit the Travis CI command