Refactor conditional logic to use a boolean connective

* Change the guard clause into a simple if/else block. I believe a
  guard clause is only useful if it happens before the majority of
  the code in the method. In this case it adds nothing, and is
  probably more difficult to understand.

lib/mutant/matcher/method.rb

@@ -54,16 +54,16 @@ module Mutant
       #
       def skip?
         location = source_location
-        if location.nil? or BLACKLIST.match(location.first)
+        if location.nil? || BLACKLIST.match(location.first)
           message = sprintf(
             '%s does not have valid source location unable to emit matcher',
             method.inspect
           )
           $stderr.puts(message)
-          return true
+          true
+        else
+          false
         end
-
-        false
       end
 
       # Return method name