ENH: Require at least ITK 5.4rc2, for ImageRandomIteratorWithIndex

Especially to avoid potential differences between registration results of the
elastix executable and ITKElastix, which were accidentally introduced with ITK
5.4rc1 and fixed with ITK 5.4rc2:

  pull request InsightSoftwareConsortium/ITK#4183
  commit InsightSoftwareConsortium/ITK@997ff54
  BUG: ImageRandomIteratorWithIndex should not assign data in constructor

setup.py

@@ -49,6 +49,6 @@
     keywords='ITK InsightToolkit',
     url=r'https://itk.org/',
     install_requires=[
-        r'itk>=5.4rc1'
+        r'itk>=5.4rc2'
     ]
     )