Drop WordPress 3.8 support

[JDGrimes]

WordPress 4.1 is now the stable version. We'll likely drop more versions in 2.0, but for now let's just drop 3.8.

Reasons include #239, and that supporting so many versions is becoming a burden on our Travis build tests. I think there are some other quirks with 3.8 as well, which I'll detail here later.

[JDGrimes]

Oh, and we should publish this on the blog to give users a "heads up".

[JDGrimes]

WP#23268 appears to be the only 3.8 other bug that we are using a deliberate workaround for.

I have noted to myself that wp_normalize_path() was introduced in 3.9, and there are a few places where we can use that in relation to module paths.

[JDGrimes]

http://wordpoints.org/archives/wordpoints-1-10-will-drop-support-for-wordpress-3-8/