travis update

.travis.yml

@@ -1,6 +1,16 @@
 language: php
+
+cache:
+  directories:
+    - $HOME/.composer/cache
+
 php:
   - '5.4'
   - '5.5'
   - '5.6'
+
+before_script:
+    - flags="--prefer-dist"
+    - composer install $flags
+
 script: phpunit