Recognize Thor files as Ruby files (#2080)

Recognize `Thorfile` and `*.thor` files
as Ruby files since they are Ruby files.

Reference:

- https://github.com/rails/thor/wiki/Getting-Started#file-names

lib/rouge/lexers/ruby.rb

@@ -11,7 +11,7 @@ class Ruby < RegexLexer
       filenames '*.rb', '*.ruby', '*.rbw', '*.rake', '*.gemspec', '*.podspec',
                 'Rakefile', 'Guardfile', 'Gemfile', 'Capfile', 'Podfile',
                 'Vagrantfile', '*.ru', '*.prawn', 'Berksfile', '*.arb',
-                'Dangerfile', 'Fastfile', 'Deliverfile', 'Appfile'
+                'Dangerfile', 'Fastfile', 'Deliverfile', 'Appfile', '*.thor', 'Thorfile'
 
       mimetypes 'text/x-ruby', 'application/x-ruby'
 

spec/lexers/ruby_spec.rb

@@ -113,6 +113,8 @@
       assert_guess :filename => 'Deliverfile'
       assert_guess :filename => 'Fastfile'
       assert_guess :filename => 'Appfile'
+      assert_guess :filename => 'Thorfile'
+      assert_guess :filename => 'foo.thor'
     end
 
     it 'guesses by mimetype' do