Populate backtrace value in request on Notify

This PR sets the backtrace request param to `stacktrace(3)` in the `airbrake.Notify` method which is the same as what is as assigned in the `airbrake.Error` method.

When backtrace isn't populated we're seeing an error response: https://gist.github.com/snormore/27856c8956bccc5421ec

@tobi @camilo