Use custom / forked http client, to avoid infinite loop due to warn-level-logging in Sentry default http client

[SimonWoolf]

See getsentry/sentry-elixir#157 . Current solution is to use a fork of sentry-elixir, which is nonideal (not least because hex package can't refer to github deps). Using a custom http client in sentry-logger-backend would be much better.