correctly invalidate course/user feature flag cache

test plan:
- change a course/user feature flag
- the change should stick and apply

Change-Id: I2b4045bf6a0c5c57c58591c98ddcd513e8243fa0
Reviewed-on: https://gerrit.instructure.com/126202
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins
Product-Review: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>

app/models/feature_flag.rb

@@ -52,7 +52,7 @@ class FeatureFlag < ActiveRecord::Base
 
   def clear_cache
     if self.context
-      self.class.connection.after_transaction_commit { MultiCache.delete(self.context.feature_flag_cache_key(feature)) }
+      self.class.connection.after_transaction_commit { self.context.feature_flag_cache.delete(self.context.feature_flag_cache_key(feature)) }
       self.context.touch if Feature.definitions[feature].try(:touch_context)
     end
   end

lib/feature_flags.rb

@@ -60,7 +60,7 @@ module FeatureFlags
   end
 
   def feature_flag_cache_key(feature)
-    ['feature_flag2', self.class.name, self.global_id, feature.to_s].cache_key
+    ['feature_flag3', self.class.name, self.global_id, feature.to_s].cache_key
   end
 
   def feature_flag_cache