Make coverage tests use development versions of lwt and lwt_ppx

[smithjessk]

See the comments starting here.

This PR fixes the Travis builds and gets the coverage tests to run. They currently don't because the tests are looking for the installed version of lwt; see e.g. here.

To verify, run make coverage.

[smithjessk]

Hmm.. The Linux builds work now, but the OSX ones fail with a different reason. Before, the build failed because lwt wasn't found. But now, it fails when installing and linking Python 2 for Opam, which wasn't installed for Opam in the old Travis build.

Not sure why this happens, but since the error occurs when installing ocaml it shouldn't be related to this PR.

[smithjessk]

Hmm.. The Linux builds work now, but the OSX ones fail with a different reason. Before, the build failed because lwt wasn't found. But now, it fails when installing and linking Python 2 for opam, which wasn't installed for opam in the old Travis build.

Not sure why this happens, but since the error occurs when installing ocaml as part of travis.sh it shouldn't be caused by this PR.

[aantron]

Thanks! I'll give it a proper try in some hours.

The macOS builds are probably failing because of the Homebrew Python issue. We worked around that in this repo, in cd63b76, however that commit is not in the ppx-expect branch. So, don't worry about macOS :)

[aantron]

Looks good, thanks again! And sorry for the delay.

I'll get this over into master shortly.

[smithjessk]

Thanks, Anton!