chore(package): update rollup-plugin-uglify to version 6.0.0

Closes #19

Author: greenkeeper[bot]

package.json

@@ -79,7 +79,7 @@
     "rollup-plugin-commonjs": "^9.1.0",
     "rollup-plugin-json": "^3.0.0",
     "rollup-plugin-node-resolve": "^3.3.0",
-    "rollup-plugin-uglify": "^4.0.0",
+    "rollup-plugin-uglify": "^6.0.0",
     "rollup-watch": "^4.3.1",
     "sinon": "^6.0.0"
   },