use latest packer binary in travis-ci
Created by: shibumi
We want to make sure that we use the latest version of packer. Currently there is no latest
-release and there will be no in the future, but we can script it. For more details see: https://github.com/hashicorp/packer/issues/5265
What needs to be done?
- modifying the travis file that it gets the latest packer binary for the syntax check.