resolve node 6 compatibility issues

package.json

@@ -66,7 +66,7 @@
     "katex": "^0.8.3",
     "marked": "^0.3.12",
     "monaco-editor": "0.13.1",
-    "monaco-editor-webpack-plugin": "^1.2.1",
+    "monaco-editor-webpack-plugin": "^1.4.0",
     "mousetrap": "^1.4.6",
     "pikaday": "^1.6.1",
     "popper.js": "^1.14.3",

yarn.lock

@@ -5272,9 +5272,9 @@ moment@2.x, moment@^2.18.1:
   version "2.19.2"
   resolved "https://registry.yarnpkg.com/moment/-/moment-2.19.2.tgz#8a7f774c95a64550b4c7ebd496683908f9419dbe"
 
-monaco-editor-webpack-plugin@^1.2.1:
-  version "1.2.1"
-  resolved "https://registry.yarnpkg.com/monaco-editor-webpack-plugin/-/monaco-editor-webpack-plugin-1.2.1.tgz#577ed091420f422bb8f0ff3a8899dd82344da56d"
+monaco-editor-webpack-plugin@^1.4.0:
+  version "1.4.0"
+  resolved "https://registry.yarnpkg.com/monaco-editor-webpack-plugin/-/monaco-editor-webpack-plugin-1.4.0.tgz#7324258ab3695464cfe3bc12edb2e8c55b80d92f"
 
 monaco-editor@0.13.1:
   version "0.13.1"