added php lint checks

Author: AndiDittrich

.travis.yml

@@ -0,0 +1,11 @@
+language: php
+php:
+    - 5.6
+    - 7.0
+    - 7.1
+    - 7.2
+    - 7.3
+    - 7.4
+
+# run lint checks
+script: find {modules,views} -name "*.php" -print0 | xargs -0 -n1 -P4 php -l