Add test for IPv6 functionality

[mcipperly]

I wrote a test for PR #138 which adds IPv6 support ( #137 )

[coveralls]

Coverage decreased (-0.8%) to 99.215% when pulling a1017fa on mcipperly:ipv6test into ace80f4 on fluent:master.

[mcipperly]

Apologies for the spam, my git is rusty and I haven't built on another PR previously!

Travis doesn't seem to support IPv6 (more info here and here ) so I did add a condition to skip in that case. Unfortunately this causes the IPv6 code to still not be covered through CI testing infrastructure - open to thoughts on how to address if this isn't workable.

[kenhys]

@mcipperly could you rebase tihis pull request?

[kenhys]

Does it need to call _is_ipv4_host() in every _make_packet?
It may be able to reuse checked status.

[mcipperly]

Updated to address the above re: not wanting to determine v4/v6 capability each time. Now:

• When we connect, save address family
• If we fail to connect in the future, reset address family and try to figure it out again - this could address issues in long-running applications where the target fluent system loses its IPv4 or IPv6 address during runtime

Also added the prefer_ipv6 option which can be set to True to prefer IPv6 and fall back to IPv4