Run go mod tidy in Go 1.12

go.mod

@@ -55,9 +55,11 @@ require (
 	github.com/gogo/protobuf v1.2.2-0.20190723190241-65acae22fc9d
 	github.com/golang/geo v0.0.0-20190916061304-5b978397cfec
 	github.com/golang/protobuf v1.3.5
+	github.com/google/btree v1.0.0 // indirect
 	github.com/google/go-cmp v0.4.0
 	github.com/google/go-github v17.0.0+incompatible
 	github.com/google/go-querystring v1.0.0 // indirect
+	github.com/gorilla/context v1.1.1 // indirect
 	github.com/gorilla/mux v1.6.2
 	github.com/gotestyourself/gotestyourself v2.2.0+incompatible // indirect
 	github.com/hailocab/go-hostpool v0.0.0-20160125115350-e80d13ce29ed // indirect