Upsert when rendering readmes

From Carol:

> Occasionally, when we change the readme rendering, I rerender all the readmes, which should update this timestamp.

Author: sgrif

src/version/mod.rs

@@ -129,9 +129,13 @@ impl Version {
 
     pub fn record_readme_rendering(&self, conn: &PgConnection) -> QueryResult<usize> {
         use schema::readme_renderings::dsl::*;
+        use diesel::dsl::now;
 
         diesel::insert_into(readme_renderings)
             .values(version_id.eq(self.id))
+            .on_conflict(version_id)
+            .do_update()
+            .set(rendered_at.eq(now))
             .execute(conn)
     }
 }