allow binding to IPv6 addresses

this fails, and is standard:
puma -b 'tcp://[2001:6e8:288::a]:9999'
this fails. and is non-standard but potentially acceptable to puma as port is mandatory argument:
puma -b 'tcp://2001:6e8:288::a:9999'

commit fixes the first issue, but is ghetto as hell, should be 1.8 .. 2.0 friendly
Merge pull request #226 from ytti/patch-1

allow binding to IPv6 addresses