bump ora2 release to 2.1.15 to fix EDUCATOR-2634

requirements/edx/github.txt

@@ -92,7 +92,7 @@ git+https://github.com/edx/django-celery.git@756cb57aad765cb2b0d37372c1855b8f5f3
 # Our libraries:
 -e git+https://github.com/edx/codejail.git@a320d43ce6b9c93b17636b2491f724d9e433be47#egg=codejail==0.0
 -e git+https://github.com/edx/acid-block.git@e46f9cda8a03e121a00c7e347084d142d22ebfb7#egg=acid-xblock
-git+https://github.com/edx/edx-ora2.git@2.1.14#egg=ora2==2.1.14
+git+https://github.com/edx/edx-ora2.git@2.1.15#egg=ora2==2.1.15
 git+https://github.com/edx/RecommenderXBlock.git@1.3#egg=recommender-xblock==1.3
 git+https://github.com/solashirai/crowdsourcehinter.git@518605f0a95190949fe77bd39158450639e2e1dc#egg=crowdsourcehinter-xblock==0.1
 -e git+https://github.com/edx/RateXBlock.git@367e19c0f6eac8a5f002fd0f1559555f8e74bfff#egg=rate-xblock