Fix Flake8 Error/Warning Parsing

Error/warning checking does not correctly parse output from flake8. This results in Syntastic incorrectly displaying some warnings as errors.

This was initially addressed by [Issue #340](https://github.com/scrooloose/syntastic/issues/340) and commit [```9504f52```](https://github.com/scrooloose/syntastic/commit/9504f526d4272d3edd567d26cefd082972fb05bd). However, the committed changes do not address all warnings/errors.