Rollback Django Debug Toolbar from 1.0.1 to 0.9.4

Due to issues with require.js and the latest version of the toolbar.
Also included a note in the requirements file explaining the situation, and what
needs to happen before this can be upgrade properly.
Merge pull request #2640 from edx/db/rollback-djdt-again

Rollback Django Debug Toolbar from 1.0.1 to 0.9.4