Multithread testing- travis

This is the second half of #732, and a companion of #754

This doesn't work with coveralls. There is some sort of error that needs to be fixed somewhere in the coveralls package. 

Coverage does work, with the nose-cov plugin. 

Any particular reason to remove `pyroma` from Travis CI? It'd be good to keep it to make sure https://github.com/python-pillow/Pillow/pull/743 passes on Travis to avoid regressions of https://github.com/python-pillow/Pillow/issues/740.