Handle NXDOMAIN correctly.

[spraints]

In our prod environment, resolv.conf includes search github.com. This leads to pages-health-check returning OK for domains like this-does-not-exist.io, because:

$ host this-does-not-exist.io
this-does-not-exist.io.github.com is an alias for github.map.fastly.net.
github.map.fastly.net has address 199.27.75.133

• failing test
• make the test fail in travis
• fix

cc @jdennes @github/pages

[spraints]

Sweet. Travis fails now, too:

Failures:
  1) GitHubPages::HealthCheck does not resolve domains that do not exist
     Failure/Error: expect(check.dns).to be_empty
       expected `[this-domain-does-not-exist-and-should-not-ever-exist.io.github.com.   29   IN   CNAME   github.map.fastly.net., github.map.fastly.net.  16      IN      A       199.27.75.133].empty?` to return true, got false
     # ./spec/github_pages_health_check_spec.rb:191:in `block (2 levels) in <top (required)>'

[jdennes]

Nice!

[benbalter]

Nice catch. 🎉