Fix usermod error on docker-based ROS Prerelease

[130s]

Without knowing, docker install instruction might have been updated to NOT use the user ubuntu. They surely updated URL of the page.

This fixes issues like ros-industrial/ros_canopen#193 (comment) (confirmed on my fork that with this change Travis passed 130s/ros_canopen#3).

[mathias-luedtke]

I would guess that Travis just migrated their VMs and just dropped the ubuntu user in favor of something else.

$USER (or even better $(whoami)) does not assume a specific username.

+1 for merge :)

[130s]

even better $(whoami)

Agreed (ref). Switched to it. Once Travis passes it's mergeable.

BTW, the section of the code that caused the error ros-industrial/ros_canopen#193 (comment) should be reached only when you enable PRERELEASE=true, which at the time of the PR is not enabled in .travis.yml. Indeed in a successful Travis build in my forked repo it's not reached. I'm not sure if there's an uncovered bug.

[mathias-luedtke]

I'm not sure if there's s uncovered bug.

I have a dedicated test-release branch which the PR was targeted at:
https://github.com/ros-industrial/ros_canopen/blob/test-release/.travis.yml

[130s]

I have a dedicated test-release branch which the PR was targeted at:
https://github.com/ros-industrial/ros_canopen/blob/test-release/.travis.yml

😌