Use slimit instead of yui for Javascript compression

[François Marier]

We should look into using slimit instead of YUI compressor to minify our Javascript:

  https://github.com/rspivak/slimit

Apparently it compresses more:

  http://ruslanspivak.com/2012/01/15/slimit-vs-yui-compressor/

[François Marier (fmarier)]

Debian request for package here:

  http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=662756

[François Marier (fmarier)]

While the slimit-compressed version of libravatar.js is marginally smaller:

-rw-r--r-- 1 francois francois 2120 jan 18 2016 libravatar.js
-rw-r--r-- 1 francois francois 896 mai 29 14:59 libravatar.js.js-yui
-rw-r--r-- 1 francois francois 876 mai 29 15:00 libravatar.js.js-slimit

The yui-compressed version of jcrop.js is much smaller:

-rw-r--r-- 1 francois francois 25936 jun 7 2014 jcrop.js
-rw-r--r-- 1 francois francois 12875 mai 29 14:59 jcrop.js.js-yui
-rw-r--r-- 1 francois francois 16997 mai 29 15:00 jcrop.js.js-slimit

Therefore let's stick with yui-compressor.