WL-3004 Upgrade raven-sentry to 4.1

This is so that when the next version of the sentry protocol is released and sentry.oucs.ox.ac.uk is upgraded we can continue to work as typically sentry only supports the 2 newest versions of the protocol.

I removed the explicit dependencies as the transitive versions are pulled in and then we don't need to keep the version numbers in sync.
Merge pull request #20 from ox-it/WL-3004

WL-3004 Upgrade raven-sentry to 4.1