Severity

Remove "fatal" severity & add payloadVersion to payload.

Related Pull Requests:
https://github.com/bugsnag/bugsnag-website/pull/189
bugsnag/bugsnag-notifier-api#3
bugsnag/bugsnag-event-worker#59
bugsnag/bugsnag-ruby#96
bugsnag/bugsnag-php#39
bugsnag/bugsnag-cocoa#14