Allow users to disable the AWS variable check
Allow users to disable the AWS variable check

This is an open source project, available to users not in the thoughtbot
organization (i.e. those without access to our S3 account).

This change makes it easier to skip the error which occurs in
bin/halcyon-build when Halcyon's AWS variables are unset.

peterwang/carnival,vaporware/carnival,thoughtbot/carnival