Add :hackney to :applications (#403)
Add :hackney to :applications (#403)

Fixes a problem where the diagnose command failed to validate push API
keys via the API, because the :hackney application wasn't loaded in
distillery releases.

This patch doesn't switch to :extra_applications to keep compatibilty
with older Elixir versions.
appsignal/appsignal-elixir,appsignal/appsignal-elixir,appsignal/appsignal-elixir,appsignal/appsignal-elixir