Seems like "since" field is introduced in the remote API 1.19.
https://docs.docker.com/reference/api/docker_remote_api_v1.19/#get-container-logs

I didn't find the "since" in API 1.18
https://docs.docker.com/reference/api/docker_remote_api_v1.18/#get-container-logs

The test for "since" pass on my local machine, where it has:
{"Version":"1.8.1","ApiVersion":"1.20","GitCommit":"d12ea79","GoVersion":"go1.4.2","Os":"linux","Arch":"amd64","KernelVersion":"4.0.9-boot2docker","BuildTime":"Thu Aug 13 02:49:29 UTC 2015"}

For the failed test on ci/circleci, it has:
{"ApiVersion":"1.18","Arch":"amd64","GitCommit":"2f3236d","GoVersion":"go1.4.2","KernelVersion":"3.13.0-61-generic","Os":"linux","Version":"1.6.2-circleci"}

The API version difference explains why. Also worth mentioning that adding the since field is a no-op for apiVersion < 1.19.

How do you suggest handling the dependency?