Fix: Travis configuration

* add some vertical whitespace
* update composer itself in before_install section
* remove --dev flag when installing dependencies with Composer as it is the default
* explicitly send builds to container-based infrastructure
* enable caching of dependencies as installed with Composer
* use phpunit as installed with Composer

Author: localheinz

.travis.yml

@@ -1,12 +1,30 @@
 language: php
+
+sudo: false
+
 php:
   - 5.5
+
+cache:
+  directories:
+    - $HOME/.composer/cache
+
 env:
   global:
     secure: Yc+Xohkr/iEUU7FCQuSLXAE9ywNW9g6CfrM1Ki0Hl+fS15F3AXT7dFY8EyCJ4dP1/oI0dBmwrGWrltXV0XWIjGV1Ms3tefCgQpBBAqwT+hImzVP3RbpZW8Iyo2d0VgiDemQF1LWYD/pKu6d8WljTnv5D77NIMdEJjQ0uzeTLWdw=
-install: composer install --dev
+
+before_install:
+  - composer self-update
+
+install:
+  - composer install --prefer-dist
+
 before_script:
   - git config --global user.email "travis-ci@codeclimate.com"
   - git config --global user.name "Travis CI"
+
+script:
+  - vendor/bin/phpunit
+
 after_script:
   - php composer/bin/test-reporter