ITK nightly version update

This commit updates:
  (1) SHA used in CMakeLists.txt to checkout ITK sources
          (InsightSoftwareConsortium/ITK@a6ec54b)
  (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/4e6beb0/scripts/update_python_version.py

Author: kwrobot

CMakeLists.txt

@@ -53,8 +53,8 @@ if(ITKPythonPackage_SUPERBUILD)
   include(ExternalProject)
 
   set(ITK_REPOSITORY "https://github.com/InsightSoftwareConsortium/ITK.git")
-  # ITK nightly-master 2017-06-06
-  set(ITK_GIT_TAG "4347f3f")
+  # ITK nightly-master 2017-06-07
+  set(ITK_GIT_TAG "a6ec54b")
 
   # Add an empty external project
   function(itk_ExternalProject_Add_Empty proj depends)

itkVersion.py

@@ -1,4 +1,4 @@
-VERSION = '4.12.0.dev20170606+12.g4347f3f'
+VERSION = '4.12.0.dev20170607+20.ga6ec54b'
 
 def get_versions():
     """Returns versions for the ITK Python package.