TravisCI should build on Linux

[dadrian]

We originally built on OS X because rocksdb was easier to install through brew. However, we end up compiling Rocksdb from source anyway, so rigging up a build system for Travis that works on Linux shouldn't be any slower.

In fact, if we can take better advantage of caching, we can probably make it even faster than the OS X build.

Furthermore, given that we deploy on Linux, we should test on Linux.