Specify dist per PHP version

Author: gnongsie

.travis.yml

@@ -4,18 +4,25 @@ matrix:
   include:
     - php: 5.6
       env: PHPUNIT_VERSION=5.6.*
+      dist: trusty
     - php: 7.0
       env: PHPUNIT_VERSION=5.6.*
+      dist: trusty
     - php: 7.1
       env: PHPUNIT_VERSION=5.7.*
+      dist: trusty
     - php: 7.2
       env: PHPUNIT_VERSION=8.5.*
+      dist: bionic
     - php: 7.3
       env: PHPUNIT_VERSION=9.5.*
+      dist: bionic
     - php: 7.4
       env: PHPUNIT_VERSION=9.5.*
+      dist: bionic
     - php: 8.0
       env: PHPUNIT_VERSION=9.5.*
+      dist: bionic
 
 sudo: false