Fix build to not produce incomplete binaries

[mumoshu]

Resolves #48

This is manually verified to work by running:

$ dist/darwin_amd64/heptio-authenticator-aws init --cluster-id foo
INFO[2018-03-19T12:09:59+09:00] generated a new private key and certificate   certBytes=811 keyBytes=1192
INFO[2018-03-19T12:09:59+09:00] saving new key and certificate                certPath=cert.pem keyPath=key.pem
INFO[2018-03-19T12:09:59+09:00] loaded existing keypair                       certPath=cert.pem keyPath=key.pem
INFO[2018-03-19T12:09:59+09:00] writing webhook kubeconfig file               kubeconfigPath=heptio-authenticator-aws.kubeconfig
INFO[2018-03-19T12:09:59+09:00] copy cert.pem to /var/heptio-authenticator-aws/cert.pem on kubernetes master node(s)
INFO[2018-03-19T12:09:59+09:00] copy key.pem to /var/heptio-authenticator-aws/key.pem on kubernetes master node(s)
INFO[2018-03-19T12:09:59+09:00] copy heptio-authenticator-aws.kubeconfig to /etc/kubernetes/heptio-authenticator-aws/kubeconfig.yaml on kubernetes master node(s)
INFO[2018-03-19T12:09:59+09:00] configure your apiserver with `--authentication-token-webhook-config-file=/etc/kubernetes/heptio-authenticator-aws/kubeconfig.yaml` to enable authentication with heptio-authenticator-aws

[nckturner]

Sorry it took so long for me to get to this! LGTM, just tested it.

[mumoshu]

@nckturner Thanks for taking your time 👍