Make runner name lowercase to work with GCE restrictions
When using the docker+machine executor with Google Compute Engine i got an error saying that the machine name wasn't allowed: Error creating machine: Error in driver during machine creation: googleapi: Error 400: Invalid value for field 'resource.name': 'runner-BEzykXoN-ci-1473082149-54819ffe'. Must be a match of regex '(?:a-z?)', invalid
To fix this I made the "runner name" lower case.