Only test PHPUnit 6.5 on PHP versions it is supported on

sebastianbergmann · sebastianbergmann · commit 27ecf6745048 · 2019-01-24T14:04:03.000+01:00
diff --git a/.travis.yml b/.travis.yml
@@ -11,11 +11,8 @@ php:
   - 7.0
   - 7.1
   - 7.2
-  - master
 
 matrix:
-  allow_failures:
-    - php: master
   fast_finish: true
 
 env:
