use rvm 2.0 and 2.1 instead of 2.0.0 and 2.1.0 at .travis.yml

See:
https://github.com/travis-ci/travis-ci/issues/2220

git-svn-id: http://svn.redmine.org/redmine/trunk@13116 e93f8b46-1217-0410-a6f0-8f06a7374b81