Remove $response->get_error_code() because return string

As in documentation this function can return string. But when we pass string as second argument in Exception class we get error.
http://codex.wordpress.org/Class_Reference/WP_Error

It Happens when "Check to confirm sending checked" because certification error.
Merge pull request #8 from babcca/patch-1

Remove $response->get_error_code() as a second argument for AirshipException. /props @babcca