Without this patch, tests would fail on IPv6 hosts with this
mysterious error:
```
_______________________________________________________________________ TestHttpMisc.test_html ________________________________________________________________________
tests/checker/test_http_misc.py:30: in test_html
self.obfuscate_test()
tests/checker/test_http_misc.py:51: in obfuscate_test
url = u"http://%s/" % iputil.obfuscate_ip(ip)
linkcheck/network/iputil.py:290: in obfuscate_ip
raise ValueError('Invalid IP value %r' % ip)
E ValueError: Invalid IP value '2a02:2e0:3fe:1001:7777:772e:2:85'
```
As it turns out, the test host (`www.heise.de`) does have an IPv6
record and our tests pass on Travis only because they do not have a
working IPv6 stack. I happen to have IPv6 at home and tests are broken
here, so add a quick workaround so tests pass again.
Ideally, we would not have to deal with this hack and would handle
"obfuscation" correctly, but I have yet to figure out what that test
actually does before fixing it properly.