Follow the naming convention for test files

'test_rules.rb' means it tests the file 'rules.rb', when in fact the file name
is 'ruleset.rb', therefore the file that contains the tests should really be
called 'test_ruleset.rb', as it is now.
Create a travis configuration file
Add rake to the gemfile

Made a dedicated 'test' group of gems. Done per
http://about.travis-ci.org/docs/user/languages/ruby/ recommendation