Make Travis cfg pretty and modular (#118)

.travis.yml

@@ -1,26 +1,44 @@
 language: rust
 
+env: RUSTFLAGS="-D warnings"
+
 matrix:
   fast_finish: true
   include:
     - rust: nightly
       os: linux
-      env: RUSTFLAGS="-D warnings" BUILD_DOCS=1 BUILD_BOOK=1
+
     - rust: nightly
       os: osx
       osx_image: xcode9.2
-      env: RUSTFLAGS="-D warnings" BUILD_DOCS=1
+
     - rust: nightly-x86_64-pc-windows-msvc
       os: windows
-      env: RUSTFLAGS="-D warnings"
 
-before_script:
-  - rustup component add rustfmt
-  - if   [[ -n "$BUILD_BOOK" ]]; then (test -x $HOME/.cargo/bin/mdbook || ./ci/install-mdbook.sh); fi
+    - name: fmt
+      rust: nightly
+      os: linux
+      before_script:
+        - rustup component add rustfmt
+      script:
+        - cargo fmt --all -- --check
+
+    - name: docs
+      rust: nightly
+      os: linux
+      script:
+        - cargo doc --features docs
+
+    - name: book
+      rust: nightly
+      os: linux
+      before_script:
+        - test -x $HOME/.cargo/bin/mdbook || ./ci/install-mdbook.sh
+        - cargo build # to find 'extern crate async_std' by `mdbook test`
+      script:
+        - mdbook build docs
+        - mdbook test -L ./target/debug/deps docs
 
 script:
-  - if ! [[ -n "$BUILD_BOOK" ]]; then cargo check --features unstable --all --benches --bins --examples --tests && cargo test --features unstable --all; fi
-  - if   [[ -n "$BUILD_BOOK" ]]; then cargo test  --features unstable --all --benches --bins --examples --tests; fi
-  - cargo fmt --all -- --check
-  - if   [[ -n "$BUILD_DOCS" ]]; then cargo doc --features docs; fi
-  - if   [[ -n "$BUILD_BOOK" ]]; then mdbook build docs && mdbook test -L ./target/debug/deps docs; fi
+  - cargo check --features unstable --all --benches --bins --examples --tests
+  - cargo test --features unstable --all