minor #77 add PHP 7.3 build job (xabbuh)

This PR was merged into the 3.0.x-dev branch.

Discussion
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add PHP 7.3 build job

Commits
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17b8d6e add PHP 7.3 build job

Author: xabbuh

.travis.yml

@@ -14,6 +14,7 @@ matrix:
   fast_finish: true
   include:
     - php: 7.1
+    - php: 7.3
     - php: 7.2
       env: deps="low"