Bump uglifier from 4.1.19 to 4.1.20 (#205)

Bumps [uglifier](https://github.com/lautis/uglifier) from 4.1.19 to 4.1.20.
- [Release notes](https://github.com/lautis/uglifier/releases)
- [Changelog](https://github.com/lautis/uglifier/blob/master/CHANGELOG.md)
- [Commits](lautis/uglifier@v4.1.19...v4.1.20)

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

Gemfile.lock

@@ -285,7 +285,7 @@ GEM
     turbolinks-source (5.2.0)
     tzinfo (1.2.5)
       thread_safe (~> 0.1)
-    uglifier (4.1.19)
+    uglifier (4.1.20)
       execjs (>= 0.3.0, < 3)
     uniform_notifier (1.12.1)
     vcr (4.0.0)