A. Johansson, Some results on colourings of graphs, Doctoral Thesis, Department of Mathematics, University of Umeå, 1994. (ISBN 91-7174-974-8)
It should also be noted that Johansson obtained this bound as a corollary of the same upper bound proved for the list-chromatic number of triangle-free graphs.
Tommy R. Jensen, September 4 1998.