Hash update: Problem Builder

Associated PRs:

- https://github.com/open-craft/problem-builder/pull/107 (Fix dashboard report image download)
- https://github.com/open-craft/problem-builder/pull/108 (Add instructions for enabling dashboard in studio)
- https://github.com/open-craft/problem-builder/pull/109 (Use dashboard image from master branch in tests)
- https://github.com/open-craft/problem-builder/pull/111 (StepBuilder: Fix inconsistent feedback for MCQs)

requirements/edx/custom.txt

@@ -11,7 +11,7 @@
 -e git+https://github.com/edx-solutions/xblock-adventure.git@7bdeb62b1055377dc04a7b503f7eea8264f5847b#egg=xblock-adventure
 -e git+https://github.com/open-craft/xblock-poll.git@bd93e4851c15d9cb1522e692f8b692cf584d23b1#egg=xblock-poll
 -e git+https://github.com/edx/edx-notifications.git@484014cc87278a5c474e6cc1bca14c417fb35599#egg=edx-notifications
--e git+https://github.com/open-craft/problem-builder.git@594f1f2853d05bf65fe4d48aeecd343088cfab4f#egg=problem-builder
+-e git+https://github.com/open-craft/problem-builder.git@8b4fd309fc78965a68205f947ddd3ac72a8f38e7#egg=problem-builder
 -e git+https://github.com/open-craft/xblock-group-project-v2.git@1913bcc962bbbf0638dc7abd99fb955fe1ace77d#egg=xblock-group-project-v2
 -e git+https://github.com/OfficeDev/xblock-officemix.git@86238f5968a08db005717dbddc346808f1ed3716#egg=xblock-officemix
 -e git+https://github.com/edx-solutions/xblock.git@d25a661454da9e6b149d6c559de50ddcd0770857#egg=xblock