Fix axis plot line length in highstock

Tick lines are drawn too long on coloraxis using highstock.

This can be reproduced replacing highcharts with the latest version of highstock in this heatmap example:
http://jsfiddle.net/gh/get/jquery/1.9.1/highslide-software/highcharts.com/tree/master/samples/highcharts/demo/heatmap-canvas/
vs
http://jsfiddle.net/pr20vbx4/

Changing axis2.len to axis2.height fixes this problem.