Update .travis.yml

Author: GrahamCampbell

.travis.yml

@@ -38,12 +38,16 @@ matrix:
       env: LARAVEL_VERSION=5.4.*
     - php: hhvm
       env: LARAVEL_VERSION=5.1.*
+      dist: trusty
     - php: hhvm
       env: LARAVEL_VERSION=5.2.*
+      dist: trusty
     - php: hhvm
       env: LARAVEL_VERSION=5.3.*
+      dist: trusty
     - php: hhvm
       env: LARAVEL_VERSION=5.4.*
+      dist: trusty
 
 before_install:
   - if [[ $TRAVIS_PHP_VERSION =~ ^hhvm ]]; then echo 'hhvm.jit = false' >> /etc/hhvm/php.ini ; fi