Replace colorize gem with rainbows

The `colorize` gem's latest version dropped support for auto-enabling
color based on whether STDOUT/STDERR were TTYs.

Switching to `rainbows` gives us this ability with relatively minor code
changes.

Change-Id: I041631b2da167ff4e6c8034f021866c2ce71bf70
Reviewed-on: http://gerrit.causes.com/36730
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>