Vault needs Java Cryptography Extensions installed

Vault's Java library needs Java Cryptography Extensions installed. This should be documented.

https://stackoverflow.com/questions/37741142/how-to-install-unlimited-strength-jce-for-java-8-in-os-x

If they are not installed or you are running too old JDK you'll get this exception:
```
java.security.InvalidKeyException: Illegal key size
```