[ANN] packetgen 2.5.0 released

packetgen 2.5.0 has been released.

* home: <https://github.com/sdaubert/packetgen>
* bugs: <https://github.com/sdaubert/packetgen/issues>
* doc: <https://github.com/sdaubert/packetgen/wiki>

PacketGen provides simple ways to generate, send and capture network
packets.

Known protocols are: Ethernet, Dot11, Dot1x, ARP, IP, IPv6, GRE, ICMP,
IGMP, ICMPv6, MLD, MLDv2, OSPFv2, OSPFv3, UDP, TCP, SNMP, ESP, DNS, IKE,
EAP, BOOTP, DHCP, TFTP
and HTTP.

Changes:

### 2.5.0 / 2018-04-02

* Add support for MLDv2, OSPFv2 and OSPFv3 protocols
* Freeze all string literals to be ready for ruby3
* Refactor some code to compute IP checksums
* Refactor some code to calculate lengths