terser is better maintained fork of uglify-js

--- a/Gemfile
+++ b/Gemfile
@@ -27,7 +27,7 @@
 
 # This needs to be with require false to avoid it being automatically loaded by
 # sprockets.
-gem "uglifier", ">= 1.3.0", require: false
+gem "terser", ">= 1.0", require: false
 
 # Explicitly avoid 1.x that doesn't support Ruby 2.4+
 gem "json", ">= 2.0.0"