Change MultipleExpectations to use 'Max' literal (rubocop#269)

Previously, MultipleExpectations depended on ConfigurableMax defining
the method `parameter_name`. This method name changed on the most recent
(unreleased) version of rubocop and this broke the cop. Probably safer
for us to rely on the literal until the public API for rubocop is clear

lib/rubocop/cop/rspec/multiple_expectations.rb

@@ -77,7 +77,7 @@ def flag_example(node, expectation_count:)
         end
 
         def max_expectations
-          Integer(cop_config.fetch(parameter_name, 1))
+          Integer(cop_config.fetch('Max', 1))
         end
       end
     end