Add support for YCbCrSubsampleRatio411 and YCbCrSubsampleRatio410

[charlievieth]

This commit adds support for the YCbCrSubsampleRatio411 and
YCbCrSubsampleRatio410 subsample ratios introduced in Go 1.5
and maintains backwards compatibility with older versions of
Go.

[unixpickle]

How is this different than #39 ?

[charlievieth]

@unixpickle Thanks for the reminder, that was so long ago I forgot I even did that and was just trying to help with #51.

Took a look at #39 and the only delta is that pre-go15 stuff is now in files named *_go14.go (which is good, because they only exist for backwards compatibility) and the tests in this PR maintain backwards compatibility in a simpler way (the current test file remains ycc_test.go - new ratios are added in ycc_go15_test.go).

Also, it appears Travis does not like this commit - likely due to test file ycc_test_go15.go not being named ycc_go15_test.go - thats now fixed.

[charlievieth]

@unixpickle I'm closing this - the changes are in: #39.