Merge pull request #440 from edx-solutions/ekolpakov/problem-builder-migration-fix

Updated problem builder hash to use version with legacy Django compatible migrations

requirements/edx/custom.txt

@@ -11,4 +11,4 @@
 -e git+https://github.com/mckinseyacademy/xblock-poll.git@ca0e6eb4ef10c128d573c3cec015dcfee7984730#egg=xblock-poll
 -e git+https://github.com/OfficeDev/xblock-officemix/@86238f5968a08db005717dbddc346808f1ed3716#egg=xblock_officemix-master
 -e git+https://github.com/edx/edx-notifications.git@1496385f16d2571bc5d958c17e30f3eac3869b8e#egg=edx-notifications
--e git+https://github.com/open-craft/problem-builder.git@dd92fd2916ba90cd9590fea0c1212b8c66236350#egg=problem-builder
+-e git+https://github.com/open-craft/problem-builder.git@1d5391df41a459d8817b513bd2f3fb670c2e18a0#egg=problem-builder