Use rustc from latest merged PR instead of nightly

.travis.yml

@@ -30,6 +30,10 @@ install:
   - npm install remark-cli remark-lint
 
 script:
+ - cargo install rustup-toolchain-install-master || echo "rustup-toolchain-install-master already installed"
+ - RUSTC_HEAD=`git ls-remote https://github.com/rust-lang/rust.git HEAD | tr -d ",." | tr " \t" "\n" | grep -e "HEAD" -v`
+ - rustup-toolchain-install-master $RUSTC_HEAD
+ - rustup default $RUSTC_HEAD
  - PATH=$PATH:./node_modules/.bin
  - remark -f README.md > /dev/null
  - set -e