Make CURLOPT_SSLVERSION configurable

[ubick]

• Allow a specific SSL/TLS version to be specified via the options array.
  Example:

use BeSimple\SoapClient\SoapClient as BeSimpleClient;

$client = new BeSimpleClient(
    'https://www.webservicex.net/geoipservice.asmx?WSDL',
    ['ssl_version' => CURL_SSLVERSION_TLSv1_2]
);