Fix predicate name style per Rubocop

lib/active_model/serializer/adapter/cache_check.rb:28:13: C: Style/PredicateName: Rename is_cached? to cached?. (https://github.com/bbatsov/ruby-style-guide#bool-methods-qmark)
        def is_cached?
            ^^^^^^^^^^
lib/active_model/serializer/adapter/cache_check.rb:32:13: C: Style/PredicateName: Rename is_fragment_cached? to fragment_cached?. (https://github.com/bbatsov/ruby-style-guide#bool-methods-qmark)
        def is_fragment_cached?
            ^^^^^^^^^^^^^^^^^^^

lib/active_model/serializer/adapter/cache_check.rb

@@ -5,11 +5,11 @@ module CacheCheck
         def cache_check(serializer)
           @cached_serializer = serializer
           @klass             = @cached_serializer.class
-          if is_cached?
+          if cached?
             @klass._cache.fetch(cache_key, @klass._cache_options) do
               yield
             end
-          elsif is_fragment_cached?
+          elsif fragment_cached?
             FragmentCache.new(self, @cached_serializer, options).fetch
           else
             yield
@@ -25,11 +25,11 @@ def cache_key
 
         private
 
-        def is_cached?
+        def cached?
           @klass._cache && !@klass._cache_only && !@klass._cache_except
         end
 
-        def is_fragment_cached?
+        def fragment_cached?
           @klass._cache_only && !@klass._cache_except || !@klass._cache_only && @klass._cache_except
         end