lock uglify-js to 2.7.x

- the new 2.8.x series breaks the `cwise` transform
- see #1450 (comment)
- maybe we should make uglify-js a core dependencies then?

Author: etpinard

package.json

@@ -131,7 +131,7 @@
     "read-last-lines": "^1.1.0",
     "requirejs": "^2.3.1",
     "through2": "^2.0.3",
-    "uglify-js": "^2.7.5",
+    "uglify-js": "~2.7.5",
     "watchify": "^3.9.0",
     "xml2js": "^0.4.16"
   }