Clean up matplotlib.colors

[alanhdu]

Various code cleanups to matplotlib.colors. There are a couple places where the logic is changed, although I'm reasonably sure the old logic was wrong:

• Original line 484 (return np.clip(lut, 0, 1) instead of just lut)
• Original line 460 and to or
• Original line 1204 value to val (which is ma.asarray(value)).

[efiring]

@alanhdu, I think the PEP-8 fixes were handled in #4155. Would you revise this, please, to include only the bug fixes, so it will merge cleanly? We would also like to see a green light from Travis.

[alanhdu]

@efiring Done.