Update minifier-css.

Thanks to xet7 !

.meteor/versions

@@ -101,7 +101,7 @@ meteorhacks:collection-utils@1.2.0
 meteorhacks:picker@1.0.3
 meteorhacks:subs-manager@1.6.4
 meteorspark:util@0.2.0
-minifier-css@1.5.0
+minifier-css@1.5.1
 minifier-js@2.6.0
 minifiers@1.1.8-faster-rebuild.0
 minimongo@1.6.0