[ANN] packetgen 3.1.1 released

packetgen 3.1.1 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.

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

Changes:

### 3.1.1 / 2018-12-21

* Types::AbstractTLV: may create aliases for type, length and/or value
accessors.
* Header::DNS::OPT#options is now a Header::DNS::ArrayOfOptions
(subclass of Types::Array).
* Header::Dot11::SubMngt#elements is now a
Header::Dot11::ArrayOfElements (subclass of Types::Array).
* Update doc for DNS.
* Bug fixes:
    * Header::DNS::Option.new#initialize accepts again :code and :data
options.