Enable the CI cache

Author: RadicalZephyr

.travis.yml

@@ -3,9 +3,11 @@ language: rust
 rust:
   - nightly
 
+cache: cargo
+
 before_script:
-  - (cargo install mdbook --version =0.1.7)
+  - (cargo install mdbook --version =0.1.7 || true)
 
-script: 
+script:
   - export PATH=$PATH:/home/travis/.cargo/bin && mdbook test
   - cd stable-check && cargo run -- ../src