Recent release of blake2 breaks argon2 rc

[tysen]

See RustCrypto/hashes#753

    Checking argon2 v0.6.0-rc.1
error[E0432]: unresolved imports `blake2::Blake2bVar`, `blake2::digest::VariableOutput`
 --> /home/ty/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/argon2-0.6.0-rc.1/src/blake2b_long.rs:6:17
  |
6 |     Blake2b512, Blake2bVar,
  |                 ^^^^^^^^^^
  |                 |
  |                 no `Blake2bVar` in the root
  |                 help: a similar name exists in the module: `Blake2bMac`
7 |     digest::{self, Digest, VariableOutput},
  |                            ^^^^^^^^^^^^^^ no `VariableOutput` in the root

[tarcieri]

Yes, working on it: RustCrypto/hashes#754