Fix sentry raven install docs
Fix sentry raven install docs

The documentation for installing Sentry's raven client was incorrect. It should state that the client must be installed with the extra flask dependencies as mentioned in getsentry/raven-python#1075
drewja/flask,drewja/flask,pallets/flask,fkazimierczak/flask,drewja/flask,mitsuhiko/flask,fkazimierczak/flask,fkazimierczak/flask,mitsuhiko/flask,pallets/flask,pallets/flask