Relax lager dependency and upgrade it to 2.0.3

This PR does two (very simple) things.

1. It relaxes the lager dependency to `"2.0.*"` instead of `"2.0.0"`
2. It upgrades to lager dependency from `2.0.0` to `2.0.3`

Without these changes (especially 1.) erlang_riemann will not work under 17.0.