Include Heroku dyno name in exception reports

[jturkel]

It would be useful to know which Heroku dyno exceptions are coming from so we can more easily correlate Bugsnag exception reports with log information.

It looks like Configuration#default_hostname explicitly doesn't return the hostname when running in Heroku which makes sense since hostnames are ephemeral. Would it be possible to instead return the dyno name when running in Heroku i.e. change the implementation of this method to ENV["DYNO"] || Socket.gethostname?