[ci] fix wrong tmp path in php-cs-fixer config (#323)

finder `exclude` paths must be relative to the `in` paths. This
prevents php-cs-fixer from attempting to scan/fix files in
`tests/tmp/cache/etc...`

Author: jrushlow

.php-cs-fixer.dist.php

@@ -7,7 +7,7 @@
 $finder = (new PhpCsFixer\Finder())
     ->in([__DIR__.'/src', __DIR__.'/tests'])
     ->exclude([
-        __DIR__.'tests/tmp'
+        'tmp'
     ])
 ;