Make use of the regex attr_reader. #164.

It would also have been valid to get rid of the attr_reader, but I like
being able to reach inside KeywordMute::Matcher without resorting to
instance_variable_get tomfoolery.

app/models/keyword_mute.rb

@@ -38,7 +38,7 @@ class KeywordMute < ApplicationRecord
     end
 
     def =~(str)
-      @regex ? @regex =~ str : false
+      regex ? regex =~ str : false
     end
   end
 end