The jsmin license is purportedly non-free because of the "...Good, not Evil" clause. It's also arguably unenforceable.
Either develop a free replacement, or investigate alternate implementations (preferably pure PHP without external dependencies, e.g., Closure or YUI compressor).
See also JsMin+ v1.4 - http://crisp.tweakblogs.net/blog/6861/jsmin%2B-version-14.html
This was fixed as side effect of #1640 - we are now using JShrink!