Swap out uglify-js for uglify-es

So we can start using ES6 dependencies without figuring out how to update babel. 

`uglify-es` is compatible with `uglify-js@3` (we were using `@2`) , which is why the same command is used. This commit includes changes to the command line to make the thing run the same as before too.

package.json

@@ -14,7 +14,7 @@
     "start:watch": "babel -s -w --skip-initial-build -d lib src",
     "start:init": "babel -s -d lib src",
     "clean": "rimraf lib dist",
-    "build": "babel -s -d lib src && rimraf dist && mkdir dist && browserify -d browser-index.js | exorcist dist/browser-matrix.js.map > dist/browser-matrix.js && uglifyjs -c -m -o dist/browser-matrix.min.js --source-map dist/browser-matrix.min.js.map --in-source-map dist/browser-matrix.js.map dist/browser-matrix.js",
+    "build": "babel -s -d lib src && rimraf dist && mkdir dist && browserify -d browser-index.js | exorcist dist/browser-matrix.js.map > dist/browser-matrix.js && uglifyjs -c -m -o dist/browser-matrix.min.js --source-map \"filename='dist/browser-matrix.min.js.map',content='dist/browser-matrix.js.map'\" dist/browser-matrix.js",
     "dist": "npm run build",
     "watch": "watchify -d browser-index.js -o 'exorcist dist/browser-matrix.js.map > dist/browser-matrix.js' -v",
     "lint": "eslint --max-warnings 102 src spec",
@@ -84,7 +84,7 @@
     "rimraf": "^2.5.4",
     "source-map-support": "^0.4.11",
     "sourceify": "^0.1.0",
-    "uglify-js": "^2.8.26",
+    "uglify-es": "^3.3.9",
     "watchify": "^3.11.0"
   },
   "browserify": {