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

Conflicts:
	.travis.yml