fix safe_yaml loading for ruby 2.2+

refs #CNVS-27229

Change-Id: I79219a24c61cb29b4ad11a74e0caa72197d2400c
Reviewed-on: https://gerrit.instructure.com/77729
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins
Product-Review: James Williams  <jamesw@instructure.com>
QA-Review: James Williams  <jamesw@instructure.com>

Author: maneframe

lib/canvas_yaml.rb

@@ -25,7 +25,11 @@
 require 'yaml'
 
 require 'syck' # so we can undo all the things before something else requires it
-YAML::ENGINE.yamler = 'psych' if defined?(YAML::ENGINE)
+if defined?(YAML::ENGINE)
+  YAML::ENGINE.yamler = 'psych'
+else
+  YAML = Psych # :/
+end
 
 require 'safe_yaml'