add default_server to listen statement for default backend

[markacola]

This will resolve #85.

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[k8s-reviewable]

This change is 

[coveralls]

Coverage remained the same at 41.305% when pulling 939c6d9 on safework:master into e58f510 on kubernetes:master.

[markacola]

I signed it!

[aledbf]

/lgtm

[aledbf]

@markacola thanks!