Move uglifyjs-webpack-plugin to ^1.1.2

Fixes uglifyJs invalid assignment.
Before we were using the uglifyjs-webpack-plugin ^0.4.6 coming
from webpack.
Took from
https://github.com/webpack-contrib/uglifyjs-webpack-plugin/issues/187

Resolves openbmc/openbmc#3200

Change-Id: I29907caeeb49122cdddf6e6793070671a4525231
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>
diff --git a/package.json b/package.json
index 6cbcc83..590f57f 100644
--- a/package.json
+++ b/package.json
@@ -101,6 +101,7 @@
     "rimraf": "^2.5.1",
     "run-sequence": "^1.1.2",
     "style-loader": "^0.19.0",
+    "uglifyjs-webpack-plugin": "^1.1.2",
     "webpack": "^3.6.0",
     "webpack-dev-server": "^2.9.1"
   },