Freeze uglify-js dependency at 1.3.4

The uglify-js npm package now points at UglifyJS2 (https://github.com/mishoo/UglifyJS), which is not API-compatible with V1.x. In particular, cabinet in its current form can not minify JS files. This patch simply freezes the version number at 1.3.4, the last released version of UgilifyJS 1.