Make Honebadger.notify() void

[subzero10]

With the introduction of async beforeNotify handlers, the return value of Honeybadger.notify() is not always consistent (you can follow discussion here at #984). To sum up:

• when async beforeNotify handlers exist, the return value of Honeybadger.notify() is always true
• when beforeNotify handlers are all synchronous, the return value indicates whether or not the error will be reported to Honeybadger

With this issue, we can make Honeybadger.notify() return void, knowing that one can know whether the error will be reported to Honeybadger with an afterNotify handler: if the callback is called with an err, it means that the error was not sent to Honeybadger.

Note: This should be a major version bump.