Update .travis.yml

Removing pylint and coverage from TravisCI. I'd like to get pylint to 0 warnings (and thus, it won't fail the build), but there's no point until we get there.

Once we are, I think it'll be perfectly valid to fail the build if warnings are introduced.

Re:coverage, there isn't much point unless they're being radiated out somehow (maybe we should look at [coveralls](https://pypi.python.org/pypi/coveralls)...)