Fix depcrecation warnings & CI

[paulgoetze]

• Updates the CI to use the latest working Elixir (1.13.4) (1.14. is not yet working, see https://github.com/deadtrickster/prometheus.ex/issues/48)

• Fixes the deprecation warning during compilation:

  warning: use Mix.Config is deprecated. Use the Config module instead

  and during test runs:

  warning: the @initial_state() notation (with parenthesis) is deprecated, please use @initial_state (without parenthesis) instead