Remove redundant cmake_minimum_required from example

This is already covered by the toplevel CMake, which also serves to
provide a consistent minimum version.

example/CMakeLists.txt

@@ -1,6 +1,4 @@
 #vim: et ts =4 sts = 4 sw = 4 tw = 0
-cmake_minimum_required(VERSION 3.1)
-
 set(EXAMPLES
     readFromString
     readFromStream