pin whisper to the 0.9.x branch from github

With the following merge:

https://github.com/graphite-project/whisper/pull/80

The 0.9.x branch of graphite-web *should* be able to use the 0.9.x branch of
whisper now again.
Merge pull request #749 from esc/requirements/fix_whisper_dependency

pin whisper to the 0.9.x branch from github