ITK nightly version update

This commit updates:
  (1) SHA used in CMakeLists.txt to checkout ITK sources
          (InsightSoftwareConsortium/ITK@df839c449b)
  (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/a93a4cc/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 2019-08-22
-  set(ITK_GIT_TAG "55967df5f3")
+  # ITK nightly-master 2019-08-23
+  set(ITK_GIT_TAG "df839c449b")
 
   #-----------------------------------------------------------------------------
   # A separate project is used to download ITK, so that it can reused

itkVersion.py

@@ -1,4 +1,4 @@
-VERSION = '5.0.1.dev20190822+297.g55967df5f3'
+VERSION = '5.0.1.dev20190823+305.gdf839c449b'
 
 def get_versions():
     """Returns versions for the ITK Python package.