Update .travis.yml

.travis.yml

@@ -56,5 +56,9 @@ script:
 after_success:
   - if [[ $CODECOVERAGE == 1 ]]; then bash <(curl -s https://codecov.io/bash); fi
 
+cache:
+  directories:
+    - $HOME/.composer/cache
+
 notifications:
   email: false