Update supported webhook version to 4

Alertmanager 0.6 bumped webhook version from 3 to 4, update code and tests

Author: prymitive

main.go

@@ -17,7 +17,7 @@ import (
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
-const supportedWebhookVersion = "3"
+const supportedWebhookVersion = "4"
 
 var (
 	addr        = "localhost:9097"

main_test.go

@@ -142,8 +142,8 @@ var amNotification = []byte(`{
 	},
 	"receiver": "alertmanager2es",
 	"status": "firing",
-	"version": "3",
+	"version": "4",
 	"groupKey": "{}/{}/{notify=\"default\":{alertname=\"Foo_Bar\", instance=\"foo\"}"
 }`)
 
-var esDocument = []byte(`{"alerts":[{"annotations":{"link":"https://example.com/Foo+Bar","summary":"Alert summary"},"endsAt":"0001-01-01T00:00:00Z","generatorURL":"https://example.com","labels":{"alertname":"Foo_Bar","instance":"foo"},"startsAt":"2017-02-02T16:51:13.507955756Z","status":"firing"}],"commonAnnotations":{"link":"https://example.com/Foo+Bar","summary":"Alert summary"},"commonLabels":{"alertname":"Foo_Bar","instance":"foo"},"externalURL":"https://alertmanager.example.com","groupLabels":{"alertname":"Foo_Bar"},"receiver":"alertmanager2es","status":"firing","version":"3","groupKey":"{}/{}/{notify=\"default\":{alertname=\"Foo_Bar\", instance=\"foo\"}","@timestamp":"2017-02-02T19:37:22+01:00"}`)
+var esDocument = []byte(`{"alerts":[{"annotations":{"link":"https://example.com/Foo+Bar","summary":"Alert summary"},"endsAt":"0001-01-01T00:00:00Z","generatorURL":"https://example.com","labels":{"alertname":"Foo_Bar","instance":"foo"},"startsAt":"2017-02-02T16:51:13.507955756Z","status":"firing"}],"commonAnnotations":{"link":"https://example.com/Foo+Bar","summary":"Alert summary"},"commonLabels":{"alertname":"Foo_Bar","instance":"foo"},"externalURL":"https://alertmanager.example.com","groupLabels":{"alertname":"Foo_Bar"},"receiver":"alertmanager2es","status":"firing","version":"4","groupKey":"{}/{}/{notify=\"default\":{alertname=\"Foo_Bar\", instance=\"foo\"}","@timestamp":"2017-02-02T19:37:22+01:00"}`)