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
BUG: Update ExecutionModelTourTest input scene to include MRML version

This commit updates the test input data to account for commit r23331
including MRML version in the mrml file.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23337 3bd1e089-480b-0410-8dfb-8563597acbee
BUG: Update VTK project to fix build using Xcode 5.1.1

This commit updates the Slicer VTK project to include the following
patch cherry-picked from current VTK development branch.

Thanks to Hans Johnson for suggesting the backport.

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Author: Sean McBride <sean@rogue-research.com>
Date:   Thu Feb 13 14:26:42 2014 -0500

    Removed -fobjc-gc from VTK_REQUIRED_OBJCXX_FLAGS

    There are 3 memory management models in Cocoa,
    chronologically by introduction: MRR, GC, ARC.
    VTK supports the first two.
    GC is now deprecated however, and Xcode 5.1 will
    no longer even build GC code.  Therefore
    removed the -fobjc-gc option as a default.
    (It built the libraries as both MRR and GC
    compatible, which was convienent for library code.)
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git-svn-id: http://svn.slicer.org/Slicer4/trunk@23338 3bd1e089-480b-0410-8dfb-8563597acbee