Spec: fix add_remove_associations count mismatch

QA-391

Test Plan:
--- Passes Jenkins

Change-Id: Id31071cee1d46684167eb11b39fe21664b4373c5
Reviewed-on: https://gerrit.instructure.com/133298
Tested-by: Jenkins
Reviewed-by: Heath Hales <hhales@instructure.com>
Product-Review: Indira Pai <ipai@instructure.com>
QA-Review: Indira Pai <ipai@instructure.com>

spec/selenium/helpers/blueprint_common.rb

@@ -149,6 +149,7 @@ module BlueprintCourseCommon
           keep_trying = true
         end
       end
+      wait_for_ajaximations
     end
 
     # reutrn the <tboey> holding the list of avaiable courses