Update rspec-core dependency to version 2.6.4
Update rspec-core dependency to version 2.6.4

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