Fix detection of uglify-js / gzip

Because of the `2>` redirect, the detection of uglify-js and gzip does not work. Here's a suggestion for a fix, would make it easier if ever there's a particular version required too, but there might be better solutions.

Encountered this as I didn't have uglify-js installed and got a strange broken pipe error. Another solution would be to simply rescue Errno::EPIPE, but I was not able to reproduce the broken pipe error with trivial examples. Also I feared it could misinterpret problems with the program with the absence of it, but I don't know much about this.