Update rustler_precompiled 0.8.4 => 0.9.0

The upgrade surfaces a dependency problem that went unnoticed.

rustler_precompiled version 0.8.4 required castore, and that satisfied
the requirements for time_zone_info (which required the dependency
despite not listing it as such). With the upgrade to rustler_precompiled
0.9.0, castore is no longer listed as a dependency.

As it's still needed by time_zone_info, it's added to mix.exs. This
should have been done before, right when time_zone_info was added.

mix.exs

@@ -112,6 +112,7 @@ defmodule MusicLibrary.MixProject do
 
       # Time-zone support - requires mint and castore
       {:time_zone_info, "~> 0.7.8"},
+      {:castore, "~> 1.0"},
 
       # Data validation
       {:nimble_options, "~> 1.1"},