install vtk5 for travis (speed up compilation process)
Fix superbuild when USE_SYSTEM_VTK is turned ON

When system VTK is used, there should not be any dependencies for ITK target to
VTK's one
compile ITK with VTK modules only if system VTK is at least vtk6
travis machines have 2 cores available

http://docs.travis-ci.com/user/ci-environment/#The-Build-Machine
Merge pull request #91 from statismo/TravisVTK

speed up compilation on travis by installing packages for vtk