ITK nightly version update

This commit updates:
  (1) SHA used in CMakeLists.txt to checkout ITK sources
          (InsightSoftwareConsortium/ITK@db6a7f2)
  (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/aebe617/scripts/update_python_version.py

Author: kwrobot

CMakeLists.txt

@@ -68,8 +68,8 @@ if(ITKPythonPackage_SUPERBUILD)
   include(ExternalProject)
 
   set(ITK_REPOSITORY "https://github.com/InsightSoftwareConsortium/ITK.git")
-  # ITK nightly-master 2017-12-10
-  set(ITK_GIT_TAG "dcf63d4")
+  # ITK nightly-master 2017-12-11
+  set(ITK_GIT_TAG "db6a7f2")
 
   #-----------------------------------------------------------------------------
   # A separate project is used to download ITK, so that it can reused

itkVersion.py

@@ -1,4 +1,4 @@
-VERSION = '4.13rc01.dev20171210+32.gdcf63d4'
+VERSION = '4.13rc01.dev20171211+38.gdb6a7f2'
 
 def get_versions():
     """Returns versions for the ITK Python package.