Drop doc publish from CI pipeline and use docs.rs

At the time of writing docs.rs wasn't a thing as of yet.  Now that
it is a (very convenient) thing, use it.

Signed-off-by: Paul Osborne <osbpau@gmail.com>

Author: posborne

.travis.yml

@@ -6,13 +6,6 @@ rust:
   - beta
   - nightly
 
-before_script:
-  - pip install 'travis-cargo<0.2' --user
-
 script:
   - cargo build --verbose
   - cargo test --verbose
-  - travis-cargo --only stable doc
-
-after_success:
-  - travis-cargo --only stable doc-upload

README.md

@@ -5,7 +5,7 @@ Rust Spidev
 [![Version](https://img.shields.io/crates/v/spidev.svg)](https://crates.io/crates/spidev)
 [![License](https://img.shields.io/crates/l/spidev.svg)](https://github.com/rust-embedded/rust-spidev/blob/master/README.md#license)
 
-[Documentation](https://rust-embedded.github.io/rust-spidev)
+[Documentation](https://docs.rs/spidev)
 
 The Rust `spidev` seeks to provide full access to the Linux spidev
 device in Rust without the need to wrap any C code or directly make