Add version to treetop installation so that things work in an omnibus setting

I couldn't get installation working in omnibus because I kept getting an "Illformed requirement" error. Apparently, if the version isn't set, it fails when creating a gem dependency. I'm not sure if the error comes up in other environments, but I'm in an omnibus one. Things seem to work fine with this, though.

There might be a more elegant fix, but I stopped here.