Correct ssl option handling

Please look at this line. https://github.com/derekcollison/nats/blob/master/lib/nats/server/options.rb#L176

When the content of options[:ssl] is "false", ssl_required should be "false", too.

But In the current version, ssl_required is "true" even if  the content of options[:ssl] is "false". The results of my test is below:

<pre>
|  config file   |  ssl_required  |
|      none      |     false      |
|     false      |      true      |
|      true      |      true      |
</pre>

I think this should be as below:

<pre>
|  config file   |  ssl_required  |
|      none      |     false      |
|     false      |     false      |
|      true      |      true      |
</pre>

I fixed it that ssl_required become "false" when options[:ssl] is "false".