Fix minor `go tool vet` warning

Was getting:

    $ go tool vet -methods -printf -rangeloops .
    worker/deployer/simple_test.go:250: possible formatting directive in Log call
Merge pull request #842 from johnweldon/go-vet-warning-fix

Fix minor `go tool vet` warning

Was getting:

    $ go tool vet -methods -printf -rangeloops .
    worker/deployer/simple_test.go:250: possible formatting directive in Log call