strongswan: Update to 5.3.1

Fixed a denial-of-service and potential remote code execution vulnerability
triggered by IKEv1/IKEv2 messages that contain payloads for the respective
other IKE version. Such payload are treated specially since 5.2.2 but because
they were still identified by their original payload type they were used as
such in some places causing invalid function pointer dereferences.
The vulnerability has been registered as CVE-2015-3991.

https://www.strongswan.org/blog/2015/06/01/strongswan-vulnerability-%28cve-2015-3991%29.html

The increased buffer size has been fixed in bug #943 upstream
  https://wiki.strongswan.org/issues/943