bump go dependencies, migrate to go1.15

go.mod

@@ -1,21 +1,14 @@
 module github.com/orange-cloudfoundry/credhub_exporter
 
-go 1.14
+go 1.15
 
 require (
-	code.cloudfoundry.org/credhub-cli v0.0.0-20200325195429-1faf152db5a4
-	github.com/alecthomas/units v0.0.0-20190924025748-f65c72e2690d // indirect
-	github.com/google/go-github/v25 v25.1.3 // indirect
-	github.com/konsorten/go-windows-terminal-sequences v1.0.2 // indirect
-	github.com/pkg/errors v0.9.1 // indirect
-	github.com/prometheus/client_golang v1.5.1
-	github.com/prometheus/common v0.9.1
-	github.com/prometheus/procfs v0.0.11 // indirect
-	github.com/prometheus/promu v0.5.0 // indirect
-	github.com/sirupsen/logrus v1.5.0 // indirect
-	golang.org/x/crypto v0.0.0-20200403201458-baeed622b8d8 // indirect
-	golang.org/x/oauth2 v0.0.0-20200107190931-bf48bf16ab8d // indirect
-	golang.org/x/sys v0.0.0-20200406113430-c6e801f48ba2 // indirect
-	google.golang.org/appengine v1.6.5 // indirect
+	code.cloudfoundry.org/credhub-cli v0.0.0-20201005130041-1bceca68b93b
+	github.com/prometheus/client_golang v1.7.1
+	github.com/prometheus/common v0.14.0
+	github.com/prometheus/procfs v0.2.0 // indirect
+	github.com/sirupsen/logrus v1.7.0 // indirect
+	golang.org/x/net v0.0.0-20201006153459-a7d1128ccaa0 // indirect
+	golang.org/x/sys v0.0.0-20201007082116-8445cc04cbdf // indirect
 	gopkg.in/alecthomas/kingpin.v2 v2.2.6
 )