Add diesel-cli to the buildpacks, don't drop schema_migrations

We'll want to drop `schema_migrations` sometime soonish, but for now
let's make sure that everything just works. We don't want to incur
downtime if we have to roll back.

Author: sgrif

.buildpacks

@@ -2,3 +2,4 @@ https://github.com/rcaught/heroku-buildpack-cmake#e4e2c9e
 https://github.com/emk/heroku-buildpack-rust#035823
 https://github.com/tonycoco/heroku-buildpack-ember-cli
 https://github.com/ryandotsmith/nginx-buildpack.git#af813ba
+https://github.com/sgrif/heroku-buildpack-diesel#43267f2

src/bin/migrate.rs

@@ -31,8 +31,7 @@ fn main() {
             conn.execute("INSERT INTO __diesel_schema_migrations (
                 SELECT version::text AS version, CURRENT_TIMESTAMP as run_on
                     FROM schema_migrations
-            )")?;
-            conn.execute("DROP TABLE schema_migrations")
+            ) ON CONFLICT DO NOTHING")?;
         }).unwrap();
     }