Add this package to edx-platform requirements

[gsidebo]

At some point we'll need to add this package (rapid-response-xblock) to a requirements file in the mitodl fork of edx-platform. requirements/edx/github.txt probably makes the most sense as the edx-sga dependency is included there in a section titled # Third Party XBlocks

[pdpinch]

I think this was done in mitodl/edx-platform#86