ITK nightly version update

This commit updates:
  (1) SHA used in CMakeLists.txt to checkout ITK sources
          (InsightSoftwareConsortium/ITK@dfc71f1)
  (2) VERSION variable set in itkVersion.py

It was automatically generated by the script update_python_version.sh [1]

[1] https://github.com/InsightSoftwareConsortium/ITKPythonPackage/blob/52cc7de/scripts/update_python_version.py

Author: kwrobot

CMakeLists.txt

@@ -73,8 +73,8 @@ if(ITKPythonPackage_SUPERBUILD)
   include(ExternalProject)
 
   set(ITK_REPOSITORY "https://github.com/InsightSoftwareConsortium/ITK.git")
-  # ITK nightly-master 2018-05-09
-  set(ITK_GIT_TAG "64703c6")
+  # ITK nightly-master 2018-05-10
+  set(ITK_GIT_TAG "dfc71f1")
 
   #-----------------------------------------------------------------------------
   # A separate project is used to download ITK, so that it can reused

itkVersion.py

@@ -1,4 +1,4 @@
-VERSION = '5.0a01.dev20180509+130.g64703c6'
+VERSION = '5.0a01.dev20180510+134.gdfc71f1'
 
 def get_versions():
     """Returns versions for the ITK Python package.