Bump spanner lib to v1.5.0 (golang-migrate#403)

* Bump spanner lib to v1.5.0

Will enable spanner emulator support
https://cloud.google.com/spanner/docs/emulator#go

Current version supports SPANNER_EMULATOR_HOST but fails at run-time
with spanner permission errors.

* go mod tidy

go.mod

@@ -1,9 +1,8 @@
 module github.com/golang-migrate/migrate/v4
 
 require (
-	cloud.google.com/go v0.53.0 // indirect
-	cloud.google.com/go/spanner v1.2.0
-	cloud.google.com/go/storage v1.5.0
+	cloud.google.com/go/spanner v1.5.0
+	cloud.google.com/go/storage v1.6.0
 	github.com/ClickHouse/clickhouse-go v1.3.12
 	github.com/Microsoft/go-winio v0.4.14 // indirect
 	github.com/aws/aws-sdk-go v1.17.7
@@ -41,11 +40,10 @@ require (
 	github.com/xdg/stringprep v1.0.0 // indirect
 	gitlab.com/nyarla/go-crypt v0.0.0-20160106005555-d9a5dc2b789b // indirect
 	go.mongodb.org/mongo-driver v1.1.0
-	golang.org/x/exp v0.0.0-20200213203834-85f925bdd4d0 // indirect
-	golang.org/x/net v0.0.0-20200202094626-16171245cfb2
-	golang.org/x/tools v0.0.0-20200213224642-88e652f7a869
-	google.golang.org/api v0.17.0
-	google.golang.org/genproto v0.0.0-20200212174721-66ed5ce911ce
+	golang.org/x/net v0.0.0-20200324143707-d3edc9973b7e
+	golang.org/x/tools v0.0.0-20200413015812-1f08ef6002a8
+	google.golang.org/api v0.21.0
+	google.golang.org/genproto v0.0.0-20200413115906-b5235f65be36
 	modernc.org/b v1.0.0 // indirect
 	modernc.org/db v1.0.0 // indirect
 	modernc.org/file v1.0.0 // indirect