Enable theft tests in Travis-CI

[silentbicycle]

It would be great to get the theft-based tests running on Travis-CI. Currently, they aren't active because we can't assume libtheft will be present and aren't building it. (greatest is used for unit tests, but theft is used for additional property-based testing / stress tests.)

Vendoring theft in the heatshrink repo would work, but I would prefer to set it up as a git subtree (or otherwise reference it externally) so upstream changes would be incorporated. (It's probably not worth setting up greatest as a sub-repo, since it's just one file. It's already included.)