Revert "driver: use simplesurance/pq instead of lib/pq"

This reverts commit ac3fb2a.

Switch back to the upstream lib/pq version.
The issue that was the cause for switching to simplesurance/pq was fixed
in lib/pq 1.8 (lib/pq#1013).

driver/postgres/postgres.go

@@ -11,7 +11,7 @@ import (
 	"github.com/simplesurance/migrate/file"
 	"github.com/simplesurance/migrate/migrate/direction"
 	"github.com/jmoiron/sqlx"
-	"github.com/simplesurance/pq"
+	"github.com/lib/pq"
 )
 
 type Driver struct {