Fix diesel_migrations feature guard
This feature guard is required when diesel is enabled but
diesel_migrations is not.
@@ -26,7 +26,7 @@ extern crate serde_json;
#[cfg(all(feature = "diesel", feature = "store"))]
extern crate diesel;
#[macro_use]
-#[cfg(feature = "diesel")]
+#[cfg(feature = "diesel_migrations")]
extern crate diesel_migrations;
#[cfg(feature = "tap")]
#[macro_use]
