Correct source map url (relative to sourcemap file)

[asottile]

Resolves #201

There's now an "output_filename_hint" which can be passed to sass.compile while in filename= mode which helps libsass make the right decisions on relative paths

[asottile]

I had to drop pypy3 (pip has dropped support) -- I also moved the job to the new trusty travis-ci distribution.

Our implementation still works with pypy3, but we have no easy way to test that in CI

[coveralls]

Coverage increased (+0.2%) to 94.473% when pulling 66de515 on correct_source_map_url into 29bb9c1 on master.

[coveralls]

Coverage increased (+0.08%) to 94.301% when pulling 8114fd9 on correct_source_map_url into 29bb9c1 on master.

[coveralls]

Coverage increased (+0.08%) to 94.301% when pulling 8114fd9 on correct_source_map_url into 29bb9c1 on master.

[coveralls]

Coverage increased (+0.2%) to 94.473% when pulling 66de515 on correct_source_map_url into 29bb9c1 on master.

[coveralls]

Coverage increased (+0.2%) to 94.473% when pulling 143ab28 on correct_source_map_url into 29bb9c1 on master.