don't use ruby-debug19 if testing against ruby-head (1.9.3)
don't use ruby-debug19 if testing against ruby-head (1.9.3)

flores/travis-ci,fxposter/travis-ci,flores/travis-ci,openeducation/travis-ci,flores/travis-ci,openeducation/travis-ci,openeducation/travis-ci,fxposter/travis-ci,fxposter/travis-ci