Fix for bug in Gem pessimistic constraint conversion

Hi Jordan - I just found a bug in the way FPM converts from Gem's pessimistic constraints.

I'm not sure if this code suits your plans but I've added a function to the Util module and have included it in the RPM class to allow it to be used there. I also added some tests.

I hope it's helpful. I really love FPM.

Cheers,
Tom