Make JMeq universe polymorphic.

This fixes #33.  This does not significantly change the time it takes to
compile coq; I have:
Before:
real    63m35.413s
user    52m31.897s
sys     10m12.110s
After:
real    63m30.077s
user    52m37.605s
sys     10m15.182s

Because of this, I don't think it hurts to make the entire file universe
polymorphic.
Merge pull request #40 from JasonGross/HoTT-JMeq-polymorphic

Make JMeq universe polymorphic.