Remove dependency on libc

[cpnielsen]

This commit simply adds CGO_ENABLED=0 to enable a completely static build. This will make the binary run without issue in containers (such as Alpine) and environments that do not bundle libc or comparable libraries.

[coveralls]

Coverage remained the same at 47.768% when pulling a15e3c6 on cpnielsen:master into d5f4bc8 on garethr:master.

[garethr]

Thanks, good tip. Appreciated.