Re-enable travis caching but with set rustfmt version.

Author: illegalprime

.travis.yml

@@ -1,8 +1,9 @@
 language: rust
 rust: nightly
+cache: cargo
 before_script:
   - export PATH="$PATH:$HOME/.cargo/bin"
-  - which rustfmt || cargo install rustfmt
+  - which rustfmt || cargo install --vers 0.8.4 rustfmt
 
 script:
     - cargo fmt -- --write-mode=diff