message: docs(readme): make docs badges more consistent
diff: [![MIT licensed](https://img.shields.io/badge/license-MIT-blue.svg)](./LICENSE)
  [![crates.io](http://meritbadge.herokuapp.com/hyper)](https://crates.io/crates/hyper)
  [![Released API docs](https://docs.rs/hyper/badge.svg)](http://docs.rs/hyper)
  -
  -([Master API docs](http://hyperium.github.io/hyper/master))
  +[![Master API docs](https://img.shields.io/badge/docs-master-green.svg)](http://hyperium.github.io/hyper/master)
  A low-level HTTP implementation for Rust.
  