Remove backwards compatibility in rijndael.

Remove compatibility for insecurely encrypted values that were created
prior to 2.3.1. If you relied on this behavior you should re-encrypt
data before upgrading.
Use /dev/urandom for random data.
Kill Security::cipher()

It was never a good idea, and is very likely to be extremely insecure.
Using proper algorithms is a better plan.
CookieComponent no longer defaults to 'cipher' mode.

With cipher removed there is no way to rely on it anymore. Use rijndael
instead.
Replace a trigger_error with an exception.
Throw exceptions instead of trigger_errors inside Security.

Exceptions are more attention grabbing than errors, and you cannot
supress them. This forces developers to solve the problems more than
errors do.
Remove Security.cipherSeed.

It is no longer used in CakePHP core and should be removed.
Merge pull request #1464 from markstory/3.0-security-improvements

3.0 security improvements

Remove insecure features and backwards compatibility shims.