Prevent concurrent minify processing

[gunjanpatidar]

We've been using minify in production for a while. Recently when a stylesheet is updated under high load many requests race to minify the stylesheet causing 100% CPU usage on all cores.

There's a similar issue at #266

but this seems to be missing in current releases

[glensc]

indeed, i have experience similar problems, and it's especially worse if you use local java process to do minifying (yui or closure compiler), especially for css as it spawns minify process for each source in a group

https://github.com/mrclay/minify/blob/2.2.0/min/lib/Minify.php#L512