MDL-43018 JavaScript: Correct joining of multiple minified JS

When minifiying JS, unnecessary semi-colons are removed - this includes the
trailing semi-colon.

Since we join multiple files together after minification, we need to add an
additional semi-colon in this situation to produce valid JavaScript.

lib/classes/minify.php

@@ -87,7 +87,7 @@ EOD;
             $compressed[] = self::js($content);
         }
 
-        return implode("\n", $compressed);
+        return implode(";\n", $compressed);
     }
 
     /**