Upgrade JCaptcha to 1.0 RC6 and use MemoryStore

We are currently using an old fork of the JCAPTCHA library (due to a licensing concerns). Upgrade to the latest version which is LGPL.

Also, use a MemoryStore rather than a disk backed EhCache store. This prevents the need for multiple JIRAs running on the same box to have a unique temp directory configured or potentially clobber each other's cache files.