Do not test on HHVM [ci skip]

I'm not sure that anyone would like to run AgenDAV on HHVM, and travis-ci
fails on phpenv under HHVM.
Use Mockery on tests [ci skip]
Add a test for our own KeboolaAesEncryptor
Add tests for SessionEncrypter
Fix warnings when testing the encryption component

We use a fake AesEncryptor class that defines an empty
__destruct() method.