Test on PHP 7.4 and all Symfony vesions (#155)

Author: GrahamCampbell

Diff for: .travis.yml

@@ -1,13 +1,48 @@
 language: php
 
-dist: xenial
-
-php:
-    - 5.6
-    - 7.0
-    - 7.1
-    - 7.2
-    - 7.3
+matrix:
+  include:
+    - php: 5.6
+      dist: xenial
+      env: SYMFONY_VERSION=^3.4
+    - php: 7.0
+      dist: xenial
+      env: SYMFONY_VERSION=^3.4
+    - php: 7.1
+      dist: bionic
+      env: SYMFONY_VERSION=^3.4
+    - php: 7.1
+      dist: bionic
+      env: SYMFONY_VERSION=^4.0
+    - php: 7.2
+      dist: bionic
+      env: SYMFONY_VERSION=^3.4
+    - php: 7.2
+      dist: bionic
+      env: SYMFONY_VERSION=^4.0
+    - php: 7.2
+      dist: bionic
+      env: SYMFONY_VERSION=^5.0
+    - php: 7.3
+      dist: bionic
+      env: SYMFONY_VERSION=^3.4
+    - php: 7.3
+      dist: bionic
+      env: SYMFONY_VERSION=^4.0
+    - php: 7.3
+      dist: bionic
+      env: SYMFONY_VERSION=^5.0
+    - php: 7.4
+      dist: bionic
+      env: SYMFONY_VERSION=^3.4
+    - php: 7.4
+      dist: bionic
+      env: SYMFONY_VERSION=^4.0
+    - php: 7.4
+      dist: bionic
+      env: SYMFONY_VERSION=^5.0
+
+before_install: composer require "symfony/process:${SYMFONY_VERSION}" --no-update
 
 install: travis_retry composer install --no-interaction --no-progress --no-suggest