fix discovery of gem binstubs

Previously, Rubinius 2 installed binstubs to:

    $(rbenv prefix)/{1.8,1.9}/bin

Now it seems that has changed to:

    $(rbenv prefix)/gems/{1.8,1.9,2.0}/bin

The tricky thing is figuring out the currently selected Rubinius mode. Very crude detection is done on the `RBXOPT=-X...` value.