Fix NameError in the Mojo lexer in Ruby 2.7 (rouge-ruby#2068)

* Fix CI for 2.7

* Use load_lexer instead

Author: bartbroere

lib/rouge/lexers/mojo.rb

@@ -3,6 +3,8 @@
 
 module Rouge
   module Lexers
+    load_lexer 'python.rb'
+    
     class Mojo < Python
       title "Mojo"
       desc "The Mojo programming language (modular.com)"