airbrake-ruby: reraise SystemExit instead of raising Airbrak::Error

Fixes #28 ("Airbrake::Error: the 'default' notifier isn't configured"
thrown after unsuccessful specs)

This fixes issues with the Rake integration. There was a [similar
issue][1] to a bug with the Rake integration reported a couple of weeks
ago. The change presented here fixes both issues by reraising
`SystemExit`, which propagates it and gets handled by Ruby.

[1]: #13

CHANGELOG.md

@@ -11,6 +11,9 @@ Airbrake Ruby Changelog
   ([#21](https://github.com/airbrake/airbrake-ruby/pull/21))
 * Made the asynchronous delivery mechanism more robust
   ([#26](https://github.com/airbrake/airbrake-ruby/pull/26))
+* Improved `SystemExit` handling by ignoring it on a different level, which
+  fixed issues with the Rake integration for the [airbrake gem][airbrake-gem]
+  gem ([#32](https://github.com/airbrake/airbrake-ruby/pull/32))
 
 ### [v1.0.2][v1.0.2] (January 3, 2016)
 

lib/airbrake-ruby.rb

@@ -280,6 +280,10 @@ def call_notifier(notifier, method, *args, &block)
       if @notifiers.key?(notifier)
         @notifiers[notifier].__send__(method, *args, &block)
       else
+        # If we raise SystemExit, the Ruby process can gracefully quit without
+        # the unwanted Airbrake::Error.
+        raise args.first if args.first.class == SystemExit
+
         raise Airbrake::Error,
               "the '#{notifier}' notifier isn't configured"
       end
@@ -289,7 +293,5 @@ def call_notifier(notifier, method, *args, &block)
 
 # Notify of unhandled exceptions, if there were any, but ignore SystemExit.
 at_exit do
-  if $ERROR_INFO && $ERROR_INFO.class != SystemExit
-    Airbrake.notify_sync($ERROR_INFO)
-  end
+  Airbrake.notify_sync($ERROR_INFO) if $ERROR_INFO
 end