build: cache .cargo/bin only

.travis.yml

@@ -1,13 +1,15 @@
 language: shell
 
-cache: cargo
+cache:
+  directories:
+    - "$HOME/.cargo/bin"
 
 before_install:
   - 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)
+install:
+  - cargo install -Z install-upgrade mdbook
 
 script:
   - mdbook test