clear plugin settings cache after transaction commit

Change-Id: I0f6a72d7e8562618dbd98ad128b55a970c532c82
Reviewed-on: https://gerrit.instructure.com/24640
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

app/models/plugin_setting.rb

@@ -120,7 +120,9 @@ class PluginSetting < ActiveRecord::Base
   end
 
   def clear_cache
-    Rails.cache.delete(PluginSetting.settings_cache_key(self.name))
+    connection.after_transaction_commit do
+      Rails.cache.delete(PluginSetting.settings_cache_key(self.name))
+    end
   end
 
   def self.encrypt(text)