Update requirements for Submit-And-Compare XBlock
Update requirements for Submit-And-Compare XBlock

This commit updates requirements/edx/github.txt to
point to the Stanford-Online fork of the
Submit-And-Compare XBlock

jbassen/edx-platform,caesar2164/edx-platform,Stanford-Online/edx-platform,caesar2164/edx-platform,jbassen/edx-platform,jbassen/edx-platform,jbassen/edx-platform,Stanford-Online/edx-platform,caesar2164/edx-platform,Stanford-Online/edx-platform,Stanford-Online/edx-platform,jbassen/edx-platform,caesar2164/edx-platform