Fixed coverage setup. [master]

[mauritsvanrees]

• There was no .coveragerc to restrict the files to check. In fact, it was in the .gitignore file.
• Travis pip installed coverage, which was already there.
• Travis used coverage.pickle2json to convert the .coverage file from v3 to v4, but we were already using v4, so that broke.

Travis also gave an error:

ERROR: Something went wrong when executing '/home/travis/build/collective/Products.PloneKeywordManager/bin/coverage html --directory=parts/test/coverage'
ERROR: Returncode: 1
ERROR: Output:
ERROR: No source for code: '/home/travis/build/collective/Products.PloneKeywordManager/parts/test/18751b0da9dbd048c270e84ef48a19d2.py'.
Aborting report output, consider using -i.

Result was that coverall reported that coverage remained the same at '?%' https://coveralls.io/builds/23424458

[coveralls]

Coverage remained the same at 80.531% when pulling 24772a820767f75ec76d0ae8af103387ccd6779e on maurits-fix-coverage-setup-master into 4bdc42c2608e43bf1603e8b00700819dc236b272 on master.