New dependency: xz-utils

Reported by @erik.zmarta

I just registered to add to this bug report.

Our docker executor was failing in the same manner with 1.5 and 1.5.2. Running the runner in debug mode and correleating the issues reported with the docker log led me to realize that we were missing xz-utils for docker to extract the image.

Installing xz-utils solved the issue for us and now 1.5.2 works nicely on our Debian 8 instance.

Check your logs for messages such as:

Sep 2 09:02:05 gitlab dockerd[1771]: time="2016-09-02T09:02:05.560379431+02:00" level=error msg="Handler for POST /v1.18/images/create returned error: exec: "xz": executable file not found in $PATH"

//Erik