Looks like the latest version of PHP Code Sniffer only works on PHP 5.4+. You should be able to work around that by changing line 19 in .travis.yml to use a different URL:
- curl -L -o phpcs.phar https://github.com/squizlabs/PHP_CodeSniffer/releases/download/2.9.1/phpcs.phar
Let me know if that works.