ENH: upgrade opencv superbuild to 2.4.13.2

use github releases link instead of sourceforge
use tar.gz instead of zip to save 4 Mo in the superbuild archive

SuperBuild/CMake/External_opencv.cmake

@@ -34,8 +34,8 @@ ADD_SUPERBUILD_CMAKE_VAR(OPENCV PNG_LIBRARY)
 
 ExternalProject_Add(OPENCV
   PREFIX OPENCV
-  URL "http://downloads.sourceforge.net/project/opencvlibrary/opencv-unix/2.4.11/opencv-2.4.11.zip"
-  URL_MD5 32f498451bff1817a60e1aabc2939575
+  URL "https://github.com/opencv/opencv/archive/2.4.13.2.tar.gz"
+  URL_MD5 8feb45a71adad89b8017a777477c3eff
   BINARY_DIR ${OPENCV_SB_BUILD_DIR}
   INSTALL_DIR ${SB_INSTALL_PREFIX}
   DOWNLOAD_DIR ${DOWNLOAD_LOCATION}