Use nightly Rust in Travis CI config snippet

The current stable rustfmt (rustfmt 0.4.1-stable) does not contain
the `--check` flag, which is used in the Travis config snippet.
Once a new stable version is released, the snippet can be switched
back to stable Rust.

Author: davidalber

README.md

@@ -134,6 +134,8 @@ A minimal Travis setup could look like this (requires Rust 1.24.0 or greater):
 
 ```yaml
 language: rust
+rust:
+- nightly
 before_script:
 - rustup component add rustfmt-preview
 script: