fix(package): pin uglifyjs-webpack-plugin to 1.2.0

Workaround for webpack-contrib/uglifyjs-webpack-plugin#241

Author: gr2m

package.json

@@ -70,6 +70,7 @@
     "standard-markdown": "^4.0.2",
     "string-to-arraybuffer": "^1.0.0",
     "typescript": "^2.6.2",
+    "uglifyjs-webpack-plugin": "1.2.0",
     "webpack": "^4.0.0-beta.1",
     "webpack-bundle-analyzer": "^2.10.0",
     "webpack-cli": "^2.0.4"