"Repaired" shebang for check_translations.rb

Most work was done in 7dc0a05a71d958918644339649ac6874c821eacb, though
`check_translation.rb` still had a hardcoded shebang. This is now
fixed. #2590 is now fully resolved too.

helper-scripts/check_translations.rb

@@ -1,4 +1,4 @@
-#!/usr/bin/ruby
+#!/usr/bin/env ruby
 
 MASTER="en"