Require specific version of uglify-js

**what is the change?:**
Removed the '^' from the npm package requirement

**why make this change?:**
I am seeing failures in CI that show `uglify-js` is returning different
output there from my local environment. To further debug this I'd like
to run CI with the exact same version of `uglify-js` that I am using
locally.

**test plan:**
push and see what CI does

**issue:**
facebook#9589

package.json

@@ -88,7 +88,7 @@
     "through2": "^2.0.0",
     "tmp": "~0.0.28",
     "typescript": "~1.8.10",
-    "uglify-js": "^2.8.29",
+    "uglify-js": "2.8.29",
     "yargs": "^6.3.0"
   },
   "devEngines": {