build: use minimal rustup profile to avoid rust-docs components

.travis.yml

@@ -1,14 +1,13 @@
-language: rust
+language: shell
 
 cache: cargo
 
-rust:
+before_install:
-  - nightly
+  - curl -sSL https://sh.rustup.rs | sh -s -- -y --default-toolchain=nightly --profile=minimal
+  - export PATH="$HOME/.cargo/bin:$PATH"
 
 before_script:
   - (cargo install mdbook --force || true)
 
 script:
-  - export PATH=$PATH:/home/travis/.cargo/bin;
   - mdbook test
-