Use stable Rust in Travis CI config snippet
Use stable Rust in Travis CI config snippet

In #2725 the Travis CI config snippet was changed to use nightly
Rust because the stable rustfmt of the time (rustfmt 0.4.1-stable)
did not contain the `--check` flag, which is used in the Travis
config snippet. The current stable rustfmt (rustfmt 0.8.2-stable)
does contain the `--check` flag, so it is now possible to use the
Travis config in the README with stable rustfmt.

aidancully/rust,graydon/rust,aidancully/rust,SingingTree/rustfmt,aidancully/rust,rust-lang/rustfmt,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,rust-lang/rustfmt,graydon/rust,SingingTree/rustfmt,aidancully/rust,graydon/rust,aidancully/rust