ITK nightly version update

This commit updates:
  (1) SHA used in CMakeLists.txt to checkout ITK sources
          (InsightSoftwareConsortium/ITK@6c07d71)
  (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/8405dea/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 2018-03-19
-  set(ITK_GIT_TAG "c1aba33")
+  # ITK nightly-master 2018-03-20
+  set(ITK_GIT_TAG "6c07d71")
 
   #-----------------------------------------------------------------------------
   # A separate project is used to download ITK, so that it can reused

itkVersion.py

@@ -1,4 +1,4 @@
-VERSION = '4.13.0.dev20180319+526.gc1aba33'
+VERSION = '4.13.0.dev20180320+533.g6c07d71'
 
 def get_versions():
     """Returns versions for the ITK Python package.