having the --2.1 allows the minitest-maven-plugin, rspec-maven-plugin to interate over all possible modes
in a consistent manner.

i.e. you can configure -Djruby.versions=1.5.6,1.6.8,1.7.13,9000.dev-SNAPSHOT -Djruby.modes=1.8,1.9,2.0,2.1
the plugin will fill the matrix as allowed.