Use IPv4 to connect to ycmd

On my machine, the ycmd is started on IPv4 address only. On the other hand, the completion module in Vim tries to connect to ycmd using IPv6 only. This results in *Resource temporarily unavailable* errors.

I do not know the reason why this happens. But I guess the Bottle framework just does not support IPv6. And the Requests module tries just the first resolved address for *localhost*.

(I have not signed the Google CLA, but I'm releasing this change as a public domain.)