Added trailing slash to excluded directories

[petoc]

PHPCodeSniffer is treating paths in ignore option as patterns, when passing path build, paths like /build/, /builds/, /building/ and so on, are all ignored, same for other default paths.
As pointed out in #201, GitLab CI is by default using /builds/ directory for running jobs, so whole contents is excluded.
Adding trailing slash should fix this behavior, but PHPCodeSniffer still matches paths like /notbuild/ and ignores them.
So there probably should be leading slash, but this could affect other tools. This seems like reasonable hotfix.
Hopefully it will not cause any side effects.
Should fix issues #200, #201 .

[petoc]

Would you prefer different approach and ignore default excludes for PHPCS, when custom config is provided?

--- a/app/Support/PhpCodeSniffer.php
+++ b/app/Support/PhpCodeSniffer.php
@@ -57,7 +57,7 @@ class PhpCodeSniffer extends Tool
             define('PHP_CODESNIFFER_CBF', $tool === 'runPHPCBF');
         }
 
-        $ignore = $this->dusterConfig->get('exclude')
+        $ignore = ! $this->hasCustomConfig() && $this->dusterConfig->get('exclude')
             ? ['--ignore=' . implode(',', $this->dusterConfig->get('exclude'))]
             : [];