chore(composer): Use composer to load mrclay/minify

BREAKING CHANGE:
The CSSMin class included via minify was renamed to CSSmin.
If you were referring to it with capital M, you'll have to
change that to lower-case m.

However, note that Elgg's dependencies are not considered
public API, so this notice is only a courtesy. Please explicitly
declare your dependencies on third party vendors, even ones that
you know Elgg already includes. We may remove or update them at any time.