agama-web-server: accept both IPv6 and IPv4 connections

[lslezak]

Problem

• IPv6 not accepted by the webserver
• curl http://ipv6-localhost:3000 does not work

Solution

• Enable using both protocols, binding to the IPv6 address actually enables using both IPv4 and IPv6 protocols (unless the dualstack is manually disabled in /proc/sys/net/ipv6/bindv6only, by default it is enabled).

Testing

• Tested manually, http://ipv6-localhost:3000 works fine

[coveralls]

coverage: 74.682%. remained the same
when pulling f638f2a on ipv4+ipv6
into fc05c32 on master.