Use a Mutex in agent instead of Thread.exclusive
Log Rails version
Log Sinatra version
Resubscribe to notifications when forked

Make sure that we have an active subscriber in the case that it's not
active any more in the parent process. Also log number of events when
the transaction is not added to the queue to make any problems with
subscribing easier to debug.
Bump to v0.9.4 [ci skip]
Merge branch 'master' into develop

Conflicts:
	CHANGELOG.md
	lib/appsignal/version.rb
Bump to v0.10.beta.2 [ci skip]
Bump to v0.10.0.beta.0 [ci skip]
Refactor Transmitter http_post spec
Merge branch 'master' into develop