Bump to v0.12.beta.18 [ci skip]
Bump to v0.12.beta.18 [ci skip]

stijnster/appsignal-ruby,appsignal/appsignal-ruby,stijnster/appsignal-ruby,appsignal/appsignal-ruby,stijnster/appsignal-ruby,matrinox/appsignal-ruby,appsignal/appsignal,appsignal/appsignal-ruby,matrinox/appsignal-ruby,appsignal/appsignal-ruby,appsignal/appsignal,matrinox/appsignal-ruby