Make sure we have valid JSON for PHPCS

[shvlv]

PHPCS config might contain <arg value="p"/>. In this case, we got invalid JSON like

$ vendor/bin/phpcs --report=json ./src/functions/boot.php 
. 1 / 1 (100%)


{"totals":{"errors":0,"warnings":0,"fixable":0},"files":{"\/src\/functions\/boot.php":{"errors":0,"warnings":0,"messages":[]}}}
Time: 72ms; Memory: 14MB

With the the -q option, we ensure progress information will be suppressed (see squizlabs/PHP_CodeSniffer#969).