FIX Memory.clear when cachedir is None
FIX Memory.clear when cachedir is None

Fix #200.

tomMoral/joblib,aabadie/joblib,joblib/joblib,joblib/joblib,karandesai-96/joblib,lesteve/joblib,aabadie/joblib,lesteve/joblib,tomMoral/joblib,karandesai-96/joblib