build: lint-check need rust-docs component

Author: tesuji

.travis.yml

@@ -1,7 +1,8 @@
-language: rust
+language: shell
 
-rust:
-  - nightly
+before_install:
+  - curl -sSL https://sh.rustup.rs | sh -s -- -y --default-toolchain=nightly --profile=minimal -c rust-docs
+  - export PATH="$HOME/.cargo/bin:$PATH"
 
 install:
   - travis_retry curl -Lf https://github.com/rust-lang-nursery/mdBook/releases/download/v0.3.1/mdbook-v0.3.1-x86_64-unknown-linux-gnu.tar.gz | tar -xz --directory=$HOME/.cargo/bin