Merge pull request #3 from songy23/mod-clean-up

Run go mod tidy.

go.mod

@@ -1,9 +1,6 @@
 module contrib.go.opencensus.io/exporter/prometheus
 
 require (
-	github.com/golang/mock v1.2.0 // indirect
-	github.com/prometheus/client_golang v0.9.3-0.20190127221311-3c4408c8b829
+	github.com/prometheus/client_golang v0.9.3-0.20190127221311-3c4408c8b829 // TODO: change the version to v0.9.2 once oc-go version is updated.
 	go.opencensus.io v0.21.0-alpha // TODO [rghetia] remove after v0.21.0 is released.
-	gopkg.in/check.v1 v1.0.0-20180628173108-788fd7840127 // indirect
-	gopkg.in/yaml.v2 v2.2.2 // indirect
 )