Don't send Private-Token headers to Sentry

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/22537

This bumps 'raven' (the Ruby gem we use to send errors to Sentry) to
version 2.0.2. We need 2.0.0 or newer to be able to sanitize HTTP
headers.

See merge request !2004
Signed-off-by: Rémy Coutable <remy@rymai.me>