Add missing .travis.yml

Author: sascha-egerer

.travis.yml

@@ -0,0 +1,17 @@
+language: php
+php:
+  - 7.1
+  - 7.2
+  - 7.3
+env:
+  - dependencies=lowest
+  - dependencies=highest
+before_script:
+  - composer self-update
+  - if [ "$dependencies" = "lowest" ]; then composer update --prefer-lowest --no-interaction; fi;
+  - if [ "$dependencies" = "highest" ]; then composer update --no-interaction; fi;
+script:
+  - vendor/bin/phing
+  - >
+    wget https://github.com/maglnet/ComposerRequireChecker/releases/download/0.2.1/composer-require-checker.phar
+    && php composer-require-checker.phar check composer.json