Support custom Sentry logger value

This pull request adds `Resque::Failure::Sentry.logger` which can be used to set a custom logger value for Resque errors in Sentry. Then, in the Sentry UI, users can filter to see only Resque errors. This is completely opt-in, no logger value will be sent by default - only if  `Resque::Failure::Sentry.logger` has been set.

In addition, the gem dependency for 'sentry-raven' was changed to ">= 0.4.6" so that this gem can be used with the latest release.

Closes gocardless/resque-sentry#1