Update DownloadProject.CMakeLists.cmake.in

Fix https://github.com/libigl/libigl/issues/1021#issuecomment-440078712

cmake/DownloadProject.CMakeLists.cmake.in

@@ -14,4 +14,5 @@ ExternalProject_Add(${DL_ARGS_PROJ}-download
                     BUILD_COMMAND       ""
                     INSTALL_COMMAND     ""
                     TEST_COMMAND        ""
+                    TLS_VERIFY          OFF
 )