Fix clippy warnings

src/migrations.rs

@@ -136,11 +136,11 @@ impl Migrations {
             return Err(MigrationError::VersionTooNew(db_version));
         }
         let mig_range = self.migrations.range((Excluded(db_version), Unbounded));
-        let mut trans = conn.transaction()?;
+        let trans = conn.transaction()?;
         let mut last_ver: MigrationVersion = NO_MIGRATIONS;
         for (ver, mig) in mig_range {
             debug!(version = ver, "applying migration version");
-            mig.up(&mut trans)?;
+            mig.up(&trans)?;
             last_ver = *ver;
         }
         if last_ver != NO_MIGRATIONS {