chore(package): update terser to version 4.2.0

Closes #59

Author: greenkeeper[bot]

package.json

@@ -32,7 +32,7 @@
     "docco": "^0.8.0",
     "mocha": "^5.0.0",
     "reify": "^0.18.1",
-    "terser": "^3.16.0"
+    "terser": "^4.2.0"
   },
   "license": "MIT",
   "engines": {