Update validates_hostname to 1.0.6 to fix a bug in parsing hexadecimal-looking domain names

validates_hostname <= 1.0.5 has a bug that breaks hostnames that could be interpreted as hexadecimal numbers. This is fixed in https://github.com/KimNorgaard/validates_hostname/pull/7

Update our version of the gem to one that includes this fix, and add a test for it.

/cc @cpallares @0x7cc

Can we get this into 8.15 do you think?

Closes #827 (closed)