Use backticks for CI_HEAD_COMMIT export
Use backticks for CI_HEAD_COMMIT export

I can't nest subshells with $(), apparently. So, I'm going to use
backticks for the outer command, and $() for the inner. I'm going to
draft an issue with Travis for this.

CiscoCloud/mantl,bitium/mantl,KaGeN101/mantl,datascienceinc/mantl,CiscoCloud/microservices-infrastructure,bitium/mantl,mantl/mantl,CiscoCloud/microservices-infrastructure,KaGeN101/mantl,sehqlr/mantl,phnmnl/mantl,datascienceinc/mantl,sehqlr/mantl,mantl/mantl,phnmnl/mantl,CiscoCloud/mantl