Update rails to fix array column default values

This should fix the currently broken MRI build. You are supposed to be able to specify array column default values with Ruby Array literals. (e.g `default: []`), and recent work on activerecord has finally [fixed the issue](https://github.com/rails/rails/blob/master/activerecord/test/cases/adapters/postgresql/array_test.rb#L50). 

Changing migrations to not use the Postgres SQL array syntax, which isn't supported by the jdbc adapter, in commit https://github.com/zooniverse/Panoptes/commit/46150e08e0de06e1978b1475f4194caef8ef5ec7 originally caused the build to break.