Ubuntu 16.04 hits EOL on April 30th

[alexmbird]

ami_id defaults to the newest Ubuntu 16.04 AMI. But 16.04 hits EOL on April 30th.

[cvrajeesh]

Fix for this PR #36 is already merged

[wpietri]

Is it already merged, @cvrajeesh? When I look at main.tf, I still see it at 16.04.