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.

Also, make explicit the command to run tests on Travis CI
Merge pull request #229 from bf4/separate_test_dev_dependencies

Separate test dev dependencies