[CMake] update minimal eigenpy version

ref. https://github.com/stack-of-tasks/eigenpy/issues/126
cc. @stonneau

CMakeLists.txt

@@ -38,7 +38,7 @@ IF(BUILD_PYTHON_INTERFACE)
 
   STRING(REGEX REPLACE "-" "_" PY_NAME ${PROJECT_NAME})
 
-  ADD_REQUIRED_DEPENDENCY("eigenpy")
+  ADD_REQUIRED_DEPENDENCY("eigenpy >= 1.6.8")
   SET(BOOST_COMPONENTS ${BOOST_COMPONENTS} python)
 ENDIF(BUILD_PYTHON_INTERFACE)