Do not test on PHP < 7.2; those are EOL

Author: cweiske

.travis.yml

@@ -1,12 +1,9 @@
 language: php
 sudo: false
 php:
-  - 5.4
-  - 5.5
-  - 5.6
-  - 7
-  - 7.0
-  - 7.1
+  - 7.2
+  - 7.3
+  - 7.4
 install:
   - pear install pear/PHP_CodeSniffer
   - phpenv rehash