Add ruby 2.0+ to travis-ci

Add 2.0 and 2.1 to .travis.yml to check compatibility with ruby 2.0 and
ruby 2.1 since it's not clear if we'll next jump to 2.0 or 2.1 if we
stick with MRI.
Merge branch 'feature/ruby-2.0+' into feature/module-cache-construction