libp2p-secio 0.21.0

Secio encryption protocol for libp2p
Documentation
# 0.21.0 [2020-08-18]

- Bump `libp2p-core` dependency.

# 0.20.0 [2020-07-01]

- Updated dependencies.
- Conditional compilation fixes for the `wasm32-wasi` target
  ([PR 1633]https://github.com/libp2p/rust-libp2p/pull/1633).

# 0.19.2 [2020-06-22]

- Updated dependencies.