Make travis test for axiom dependence

Now travis will enforce the convention that nothing in the library
should depend on FunextAxiom or UnivalenceAxiom.
Add some instances about truncation of IsTrunc
Merge pull request #398 from JasonGross/istrunc-instances

Add some instances about truncation of IsTrunc
Merge pull request #397 from JasonGross/check-no-axiom

Make travis test for axiom dependence