TravisCI: Drop support for PHP 5.6 and add PHP 7.2

See #3 for decision details

.travis.yml

@@ -1,15 +1,19 @@
 language: php
+
 php:
-  - '5.6'
   - '7.0'
   - '7.1'
+  - '7.2'
   - nightly
+
 matrix:
   allow_failures:
   - php: nightly
+
 install:
   - pear install PHP_CodeSniffer
   - phpenv rehash
+
 script:
   - find . -name '*.php' -print0 | xargs -0 -n1 php -lf
   - phpcs --standard=PSR1,PSR2 --extensions=php --ignore="^$(pwd)/ext_scripts/*" . || true # Allow failures for now