chore(travis): Enable caching of ./target folder.

Travis supports the caching of folders between builds.
By storing the ./target folder we can cache hyper's dependencies.

.travis.yml

@@ -1,6 +1,10 @@
 language: rust
 sudo: false
 
+cache:
+  directories:
+      - target
+
 script:
     - cargo build
     - cargo test