Update elixir min version

[nickgartmann]

phoenix_live_reload 1.5 uses Code.loaded?, which was introduced in Elixir 1.15.0. Bump the min version to 1.15.0 to accommodate this change.

This could also be fixed by setting the phoenix_live_reload version def to something like "~> 1.2.0 and < 1.5.0"