travis: run on both OSX and Linux
travis: run on both OSX and Linux

Previously we used Docker to build on many Linux distros -- this is
slightly pointless, since all we really care about is:

- does the code compile and work on OSX
- does the code compile safely on Linux (it won't work because of a
  missing API but this is a runtime failure)

Signed-off-by: David Scott <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@recoil.org>

djs55/ocaml-osx-dnssd