Bump uglify-js from 3.1.3 to 3.4.9

Bumps [uglify-js](https://github.com/mishoo/UglifyJS2) from 3.1.3 to 3.4.9.
- [Release notes](https://github.com/mishoo/UglifyJS2/releases)
- [Commits](mishoo/UglifyJS@v3.1.3...v3.4.9)

Signed-off-by: dependabot[bot] <support@dependabot.com>

package.json

@@ -43,7 +43,7 @@
     "sass-loader": "6.0.6",
     "showdown": "^1.8.6",
     "style-loader": "0.16.1",
-    "uglify-js": "3.1.3",
+    "uglify-js": "3.4.9",
     "uikit": "3.0.0-rc.26",
     "underscore": "1.8.3",
     "vue": "2.5.21",