Add cache to .travis.yml

Author: mischov

.travis.yml

@@ -19,4 +19,10 @@ sudo: false
 
 before_install:
   - curl https://sh.rustup.rs | sh -s -- -y
-  - export PATH="$HOME/.cargo/bin:$PATH"
+  - export PATH="$HOME/.cargo/bin:$PATH"
+
+cache:
+  directories:
+    - _build
+    - deps
+    - $HOME/.cargo