Avoid rescuing from errors in Active Record transaction callbacks in versions of Rails where they will be re-raised

[imjoehaines]

Goal

Rails used to swallow errors in transaction callbacks like after_commit. In Rails 4.2, the raise_in_transactional_callbacks option was added, which make Rails re-raise errors in these callbacks. The option was removed in Rails 5 and from then on Rails will always re-raise errors in transaction callbacks

We have an Active Record integration that rescues errors in transaction callbacks and sends them to Bugsnag. This is unnecessary on Rails 5+ & 4.2 with raise_in_transactional_callbacks enabled and can cause errors to be reported twice on these versions

This PR is #709 with the config option renamed to raise_in_transactional_callbacks as that's what was eventually released in Rails 4.2 (it went through several refactors/renames across a few PRs). I've also added tests that raise errors in an after_commit callback, so we have coverage of:

• Rails 3
• Rails 4 with raise_in_transactional_callbacks enabled
• Rails 4 with raise_in_transactional_callbacks disabled
• Rails 5
• Rails 6