Cog exits with "multiple chat providers specified"

[ohaiwalt]

It appears that with the new variables set up, the fact that COG_SLACK_ENABLED and COG_HIPCHAT_ENABLED both exist with some value is causing confusion for the bot.

Apply database migrations...
+ echo 'Apply database migrations...'
+ mix ecto.migrate --no-deps-check
** (Mix.Config.LoadError) could not load config config/config.exs
    ** (RuntimeError) Multiple chat providers were specified. You can only enable one provider at a time.
    config/helpers.exs:49: Cog.Config.Helpers.enabled_chat_provider/0
    (stdlib) erl_eval.erl:670: :erl_eval.do_apply/6
    (stdlib) erl_eval.erl:878: :erl_eval.expr_list/6
    (stdlib) erl_eval.erl:236: :erl_eval.expr/5
    (stdlib) erl_eval.erl:228: :erl_eval.expr/5
    (stdlib) erl_eval.erl:229: :erl_eval.expr/5
    (stdlib) erl_eval.erl:878: :erl_eval.expr_list/6
    (stdlib) erl_eval.erl:236: :erl_eval.expr/5