Caches should not be serialized

Hello!
I faced this bug https://github.com/JodaOrg/joda-time/issues/46 too.

 Caused by: java.io.NotSerializableException: org.joda.time.tz.CachedDateTimeZone$Info
 	at java.io.ObjectOutputStream.writeNewObject(ObjectOutputStream.java:1364)
 	at java.io.ObjectOutputStream.writeObjectInternal(ObjectOutputStream.java:1671)
 	at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1517)
 	at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1481)
 	at java.io.ObjectOutputStream.writeNewArray(ObjectOutputStream.java:1205)
 	at java.io.ObjectOutputStream.writeObjectInternal(ObjectOutputStream.java:1662)
 	at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1517)
 	at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1481)
 	at java.io.ObjectOutputStream.writeFieldValues(ObjectOutputStream.java:979)
 	at java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:368)
 	at java.io.ObjectOutputStream.writeHierarchy(ObjectOutputStream.java:1074)
 	at java.io.ObjectOutputStream.writeNewObject(ObjectOutputStream.java:1404)
 	at java.io.ObjectOutputStream.writeObjectInternal(ObjectOutputStream.java:1671)
 	at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1517)
 	at java.io.ObjectOutputStream.writeObject(ObjectOutputStream.java:1481)
 	at java

As I can see it is caused by trying to serialize cache field of non serializable class. I think that it should not be serialized.
My fix passes all tests and solves this problem.
Thank you in advance for reviewing it.