rake validate learned how to force colorization

In Jenkins, there is no tty and puppet does not output any color at all.
We have a plugin in Jenkins that let us generate the HTML counterpart of
ANSI escape codes.  That will make the console output slighlty nicer.

Jenkins change is https://gerrit.wikimedia.org/r/#/c/77376/

Change-Id: Ic1adcd887a18afb146c45b5d2d04bad682dbac57
Revert "rake validate learned how to force colorization"

sorry :(

This reverts commit 078e531255ef4008d7bab357eeab73db1cdf677a.

Change-Id: Id4f546f7e1a697669781720a1705aacc56d56e4e