Fix required xeus version (#79)

Author: alexander-penev

CMakeLists.txt

@@ -79,7 +79,7 @@ endif()
 # ============
 
 set(xtl_REQUIRED_VERSION 0.7.0)
-set(xeus_REQUIRED_VERSION 3)
+set(xeus_REQUIRED_VERSION 3.0...<4.0)
 
 if (NOT TARGET xtl)
     find_package(xtl ${xtl_REQUIRED_VERSION} REQUIRED)