Use different `coveralls` lib

`python-coveralls` didn't submit information on the existing files for me.
Cf https://github.com/z4r/python-coveralls/issues/21.
Merge pull request #6 from 0x64746b/feature/use_different_coveralls_lib

Use different `coveralls` lib
Merge remote-tracking branch 'origin/master' into feature/add_tests

Conflicts:
	requirements/tests.txt