chore: updated prometheus and alertmanager dependencies (#4)

Updated versions:
* github.com/prometheus/alertmanager -> v0.20.0
* github.com/prometheus/client_golang -> v1.6.0

Co-authored-by: Vlk, Ondrej <[email protected]>

Author: FuFikCZ

CHANGELOG.md

@@ -1,5 +1,7 @@
 
 ## Unreleased
+### Changed
+- prometheus and alertmanager dependency versions
 
 ## 0.7.0 / 2019-08-13
 

go.mod

@@ -4,10 +4,10 @@ go 1.12
 
 require (
 	github.com/alecthomas/kingpin v2.2.6+incompatible
-	github.com/go-kit/kit v0.8.0
+	github.com/go-kit/kit v0.9.0
 	github.com/gorilla/mux v1.7.2
-	github.com/pkg/errors v0.8.0
-	github.com/prometheus/alertmanager v0.17.1-0.20190619131440-bef850ac905c
-	github.com/prometheus/client_golang v1.0.0
+	github.com/pkg/errors v0.8.1
+	github.com/prometheus/alertmanager v0.20.0
+	github.com/prometheus/client_golang v1.6.0
 	github.com/xanzy/go-gitlab v0.18.0
 )