Add PHP 7.3 to CI

Author: wol-soft

.travis.yml

@@ -9,6 +9,7 @@ env:
 php:
   - 7.1
   - 7.2
+  - 7.3
 
 install:
   # Install coveralls.phar
@@ -36,4 +37,4 @@ after_script:
 cache:
   directories:
     - vendor
-    - $COMPOSER_CACHE_DIR
+    - $COMPOSER_CACHE_DIR