Update .travis.yml (thephpleague#535)

Author: andreybolonin

.travis.yml

@@ -5,6 +5,7 @@ php:
   - 7.0
   - 7.1
   - 7.2
+  - 7.3
 
 # This triggers builds to run on the new TravisCI infrastructure.
 # See: http://docs.travis-ci.com/user/workers/container-based-infrastructure/