Update .travis.yml

Author: barryvdh

.travis.yml

@@ -1,17 +1,20 @@
 language: php
 
-dist: precise
-
 php:
-  - 5.3
-  - 5.4
-  - 5.5
   - 5.6
   - 7.0
   - 7.1
+  - 7.2
+
+sudo: false
+
+## Cache composer
+cache:
+  directories:
+    - $HOME/.composer/cache
 
 before_script:
-  - travis_retry composer install --no-interaction --prefer-source
+  - travis_retry composer install --no-interaction --prefer-dist
 
 script:
   - vendor/bin/phpunit