Add IPv6 support to the UDP sender

[ferrous26]

And set the socket host and port at init time so that it does not have to make a DNS call for every send.

Also update dependencies and clear warnings so that things work smoothly on Ruby 2.3.0.

[milgner]

Hi @ferrous26,

support for IPv6 is very welcome! Nonetheless I'm a bit uncomfortable with the rest of the changes as this drops support for quite a number of old Ruby versions and seems to pull in a dependency on ActiveSupport. Also, we should provide a possibility to make the connect call on the socket optional to prevent breaking it for setups that use special DNS setups like round-robin.
If you could rebase it onto the current master branch (which already addresses a number of changes you made here, too), it would be much appreciated.

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