travis: remove ~/.xargo from the cache

Author: japaric

.travis.yml

@@ -70,10 +70,7 @@ script:
 
 after_script: set +e
 
-cache:
-  cargo: true
-  directories:
-    - $HOME/.xargo
+cache: cargo
 
 before_cache:
   - chmod -R a+r $HOME/.cargo;