Adds build matrix to test against multiple versions.

.travis.yml

@@ -1,21 +1,46 @@
 language: php
 
-php:
-  - 7.0
-  - 7.1
-  - 7.2
-
 # This triggers builds to run on the new TravisCI infrastructure.
 # See: http://docs.travis-ci.com/user/workers/container-based-infrastructure/
 sudo: false
 
+matrix:
+  include:
+    - php: 7.0
+      env: LARAVEL='5.5.*' TESTBENCH='3.5.*'
+    - php: 7.1
+      env: LARAVEL='5.5.*' TESTBENCH='3.5.*'
+    - php: 7.1
+      env: LARAVEL='5.6.*' TESTBENCH='3.6.*'
+    - php: 7.1
+      env: LARAVEL='5.7.*' TESTBENCH='3.7.*'
+    - php: 7.2
+      env: LARAVEL='5.5.*' TESTBENCH='3.5.*'
+    - php: 7.2
+      env: LARAVEL='5.6.*' TESTBENCH='3.6.*'
+    - php: 7.2
+      env: LARAVEL='5.7.*' TESTBENCH='3.7.*'
+    - php: 7.3
+      env: LARAVEL='5.5.*' TESTBENCH='3.5.*'
+    - php: 7.3
+      env: LARAVEL='5.6.*' TESTBENCH='3.6.*'
+    - php: 7.3
+      env: LARAVEL='5.7.*' TESTBENCH='3.7.*'
+  fast_finish: true
+
 cache:
   directories:
-    - vendor
     - $HOME/.composer/cache
 
+before_script:
+  - composer config discard-changes true
+
+before_install:
+  - travis_retry composer self-update
+  - travis_retry composer require "laravel/framework:${LARAVEL}" "orchestra/testbench:${TESTBENCH}" --no-interaction --no-update
+
 install:
-  - travis_retry composer install
+  - travis_retry composer install --prefer-dist --no-interaction --no-suggest
 
 script:
   - vendor/bin/phpcs --standard=psr2 src/