REFAC: set minimum version of opencv to 3.0.0

Modules/ThirdParty/OpenCV/otb-module-init.cmake

@@ -21,9 +21,8 @@
 find_package ( OpenCV REQUIRED )
 mark_as_advanced( OpenCV_DIR )
 
-if("${OpenCV_VERSION}" VERSION_LESS "2.4.0")
-  message(WARNING "The OpenCV version detected (${OpenCV_VERSION}) is older than 2.4.0, "
-    "it may not be fully compatible with OTB")
+if("${OpenCV_VERSION}" VERSION_LESS "3.0.0")
+  message(ERROR " The OpenCV version detected (${OpenCV_VERSION}) is older than the minimum supported version (3.0.0) ")
 endif()
 
 if(OpenCV_VERSION_MAJOR EQUAL 4)