Chain the existing panic_hook onto Rollbar reporting

[shaunrs]

Currently the macro report_panics! works just fine, but it replaces the default panic hook.

This causes a number of issues:

1. No feedback on the console once rollbar is enabled, which is the default expected behaviour for panic!
2. A custom panic_hook that is in place before rollbar is called will no longer be called

This PR fixes that by chaining the original panic hook on the end of rollbar reporting.