Update build status to target master (#826)
Update build status to target master (#826)

Instead of targeting the latest build, which could be any PR, we should target current master results.
joehandzik/alertmanager,tyrken/alertmanager,GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,joehandzik/alertmanager,Kellel/alertmanager,Aloomaio/alertmanager,tinytub/alertmanager,Kellel/alertmanager,GoogleCloudPlatform/alertmanager,joehandzik/alertmanager,tyrken/alertmanager,Aloomaio/alertmanager,prometheus/alertmanager,tyrken/alertmanager,tinytub/alertmanager,Aloomaio/alertmanager,Kellel/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,tinytub/alertmanager,tyrken/alertmanager,prometheus/alertmanager,prometheus/alertmanager