Revert "ENH: Update Qt4 minimum required version to 4.7"

This reverts commit 2c94808a272545ae199bccf7e2c30d7e75d03c0d.

CMake/ctkMacroSetupQt.cmake

@@ -57,7 +57,7 @@ macro(ctkMacroSetupQt)
     endif()
 
   else()
-    set(minimum_required_qt_version "4.7")
+    set(minimum_required_qt_version "4.6")
 
     find_package(Qt4)