Use travis to build and test in 32-bit mode

[jberryman]

Hopefully we can do this with something like:

https://github.com/travis-ci/travis-ci/issues/5770#issuecomment-197771661

We get a lot of coverage of 32-bit consistency with GHCJS, but can't rely on that (e.g. we need to test that Word64 are stored in the way we expect, etc). And see #8 and https://github.com/jberryman/hashabler/pull/7