fix: dependencies

go.mod

@@ -0,0 +1,13 @@
+module github.com/caarlos0/openvpn_exporter
+
+require (
+	github.com/beorn7/perks v0.0.0-20180321164747-3a771d992973 // indirect
+	github.com/gogo/protobuf v1.1.1 // indirect
+	github.com/golang/protobuf v1.2.0 // indirect
+	github.com/matttproud/golang_protobuf_extensions v1.0.1 // indirect
+	github.com/prometheus/client_golang v0.9.1
+	github.com/prometheus/client_model v0.0.0-20180712105110-5c3871d89910 // indirect
+	github.com/prometheus/common v0.0.0-20181020173914-7e9e6cabbd39 // indirect
+	github.com/prometheus/procfs v0.0.0-20181005140218-185b4288413d // indirect
+	golang.org/x/sync v0.0.0-20181108010431-42b317875d0f // indirect
+)