This fixes the test failures on Travis CI.
Quoting from 07a4ff79d2
/* Set expected tag value. A restriction in OpenSSL 1.0.1c and earlier
* required the tag before any AAD or ciphertext */
...complete with tests and benchmark.
This will allow us to get rid of the dependency to spacemonkeygo/openssl
that causes problems on Arch Linux
( https://github.com/rfjakob/gocryptfs/issues/21 )