Modify TravisCI caching policy

.travis.yml

@@ -1,5 +1,5 @@
 language: rust
-cache: cargo
+
 matrix:
   include:
     # Stable channel.
@@ -103,3 +103,10 @@ deploy:
 notifications:
   email:
     on_success: never
+
+# Caching
+cache:
+  directories:
+    - /home/travis/.cargo
+before_cache:
+  - rm -rf /home/travis/.cargo/registry