COMP: Change default version of VTK to 6

As discussed during June 17 developer hangout and also highlighted on the
Slicer developer list [1], consider that a lot of developer have been
reporting issue and we now have the Slicer application able to build and
run when build against VTK6. This commit changes the default version of VTK.

It is anticipated that some extension will fail to build, error will be
addressed as they come. Additional help will also be provided for
extension developers during the coming project week. See [2]

[1] http://slicer-devel.65872.n3.nabble.com/VTK-6-enabled-by-default-tt4031841.htm

[2] http://www.na-mic.org/Wiki/index.php/2014_Summer_Project_Week

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23336 3bd1e089-480b-0410-8dfb-8563597acbee