Test against multiple Coq versions

[tchajed]

The only thing blocking this is that some examples demo coq master features, and these shouldn't be run on older versions. We can probably somehow set this up (without branches), maybe using a travis environment variable to decide what targets to build.

[JasonGross]

If you make multiple makefile targets for the different versions of Coq, you can just have a travis environment variable that says which make target to build.