BUG: remove itkVersorTransformOptimizerv4 from BRAINSTools

The VersorTransformOptimizerv4 is removed from ITKv4, and its usage 
should be replaced by newly added RegularStepGradientDescentOptimizerv4.

This patch addresses the above changes, so now we can build BRIASTools
based on the last version of ITK without any compiler error.