Update hash of xblock-utils to the latest version.

That version is required by Problem Builder.

Also remove the duplicate entry for xblock-utils in requirements/edx/custom.txt.

requirements/edx/custom.txt

 # Custom requirements to be customized by individual OpenEdX instances
 
-# When updating a hash of an XBlock that users xblock-utils, please update its version hash in github.txt
--e git+https://github.com/edx/xblock-utils.git@3b58c757f06943072b170654d676e95b9adb37b0#egg=xblock-utils
+# When updating a hash of an XBlock that uses xblock-utils, please update its version hash in github.txt.
 -e git+https://github.com/edx-solutions/xblock-mentoring.git@bd0b3f413ae7e8274985555adfd7de7af3eca84c#egg=xblock-mentoring
 -e git+https://github.com/edx-solutions/xblock-image-explorer.git@21b9bcc4f2c7917463ab18a596161ac6c58c9c4a#egg=xblock-image-explorer
 -e git+https://github.com/edx-solutions/xblock-drag-and-drop.git@92ee2055a16899090a073e1df81e35d5293ad767#egg=xblock-drag-and-drop

requirements/edx/github.txt

@@ -50,8 +50,8 @@ git+https://github.com/edx/ease.git@release-2015-07-14#egg=ease==0.1.3
 -e git+https://github.com/edx/edx-search.git@release-2015-07-03#egg=edx-search
 -e git+https://github.com/edx/edx-milestones.git@release-2015-06-17#egg=edx-milestones
 git+https://github.com/edx/edx-lint.git@ed8c8d2a0267d4d42f43642d193e25f8bd575d9b#egg=edx_lint==0.2.3
-# Note for the next rebase: custom.txt or one of XBlocks installed there might require a newer version of xblock-utils - please check versions
--e git+https://github.com/edx/xblock-utils.git@588f7fd3ee88847c57cf09d10e81caa6b267ec51#egg=xblock-utils
+# Note for the next rebase: one of XBlocks in custom.txt might require a newer version of xblock-utils - please check versions.
+-e git+https://github.com/edx/xblock-utils.git@b4f9b51146c7fafa12f41d54af752b8f1516dffd#egg=xblock-utils
 -e git+https://github.com/edx-solutions/xblock-google-drive.git@138e6fa0bf3a2013e904a085b9fed77dab7f3f21#egg=xblock-google-drive
 -e git+https://github.com/edx/edx-reverification-block.git@a286e89c73e1b788e35ac5b08a54b71a9fa63cfd#egg=edx-reverification-block
 git+https://github.com/edx/ecommerce-api-client.git@1.0.0#egg=ecommerce-api-client==1.0.0