Only run phpstan once

.travis.yml

@@ -23,15 +23,12 @@ matrix:
     - php: 5.6
     - php: 7.0
     - php: 7.1
-      env: PHPSTAN=1
     - php: 7.2
-      env: PHPSTAN=1
     - php: 7.3
       env: PHPSTAN=1
     - php: 7.3
       env:
         - deps=high
-        - PHPSTAN=1
     - php: nightly
   fast_finish: true
   allow_failures: