fix resetting courses with integration ids

test plan:
* create a course
* give it an integration id through the api
* reset the course content from the settings page
* should work

closes #CNVS-17370

Change-Id: I66d935e4688e9854a6f47604e33e358d7a0308fa
Reviewed-on: https://gerrit.instructure.com/61003
Tested-by: Jenkins
Reviewed-by: Dan Minkevitch <dan@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>

app/models/course.rb

@@ -2558,7 +2558,7 @@ class Course < ActiveRecord::Base
       # get its id to move over sections and enrollments.  Setting this course to
       # deleted has to be last otherwise it would set all the enrollments to
       # deleted before they got moved
-      self.uuid = self.sis_source_id = self.sis_batch_id = nil;
+      self.uuid = self.sis_source_id = self.sis_batch_id = self.integration_id = nil;
       self.save!
       Course.process_as_sis { new_course.save! }
       self.course_sections.update_all(:course_id => new_course)