TASK: Travis should install vendor dependencies before build

.travis.yml

 language: php
+
+cache:
+  directories:
+    - $HOME/.composer/cache
+
 php:
   - '5.4'
   - '5.5'
   - '5.6'
   - '7.0'
   - '7.1'
+
+install:
+  - 'composer install --prefer-dist'