Set sane default values for SSL settings

[efcasado]

This is a small pull-request that aims at setting sane defaults for KafkaEx's SSL settings.

The current implementation requires KafkaEx to explicitly set :use_ssl to either true or false. Otherwise, KafkaEx yields an error at boot time.

iex -S mix
Erlang/OTP 20 [erts-9.1.3] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:10] [hipe] [kernel-poll:false] [dtrace]

=INFO REPORT==== 2-Apr-2018::14:47:51 ===
    application: logger
    exited: stopped
    type: temporary
** (Mix) Could not start application kafka_ex: exited in: KafkaEx.start(:normal, [])
    ** (EXIT) an exception was raised:
        ** (FunctionClauseError) no function clause matching in KafkaEx.Config.ssl_options/2
            (kafka_ex) lib/kafka_ex/config.ex:39: KafkaEx.Config.ssl_options(nil, nil)
            (kafka_ex) lib/kafka_ex.ex:490: KafkaEx.build_worker_options/1
            (kafka_ex) lib/kafka_ex.ex:68: KafkaEx.create_worker/2
            (kafka_ex) lib/kafka_ex.ex:539: KafkaEx.start/2
            (kernel) application_master.erl:273: :application_master.start_it_old/4

The only required change would be L19. However, I took the liberty to set :ssl_options default value to an empty list, which is what the comments suggest.

# ssl_options should be an empty list by default if use_ssl is false

By doing this, we can safely remove the following ssl_option/2's function clause, which becomes dead code.

defp ssl_options(false, nil), do: []

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[joshuawscott]

Thanks!