Add Travis for CI builds

Author: rdohms

.travis.yml

@@ -0,0 +1,18 @@
+language: php
+php:
+    - '7.2'
+    - '7.3'
+    - nightly
+
+before_script:
+    - travis_retry composer self-update
+    - travis_retry composer install --no-interaction
+
+script:
+    - vendor/bin/phpunit
+    - vendor/bin/phpcs
+    - vendor/bin/phpstan analyse
+
+matrix:
+    allow_failures:
+        - php: nightly