Add missing ast with pear for phan on Travis CI

Author: vukanac

.travis.yml

@@ -11,6 +11,7 @@ cache:
 
 install:
   - composer install --no-interaction --prefer-source
+  - pecl install ast
 
 before_script:
   - wget https://github.com/infection/infection/releases/download/0.8.0/infection.phar