Get Travis build passing with linting

[tdmalone]

I mightn't be understanding what's required to test the standard on itself - and maybe we can't do this?

https://travis-ci.org/ChromatixAU/phpcs-config-chromatix/jobs/296552117#L516-L518

> bash -c 'if [[ $OSTYPE == linux* ]]; then php vendor/bin/phpcs --config-set installed_paths ../../chromatix,../../wp-coding-standards/wpcs; fi'
Directory name must not be empty.

From running vendor/bin/phpcs directly locally, this appears to be coming from https://github.com/squizlabs/PHP_CodeSniffer/blob/b5d57ed4ab5162bdc776cdc299952ac6986c0177/src/Util/Standards.php#L93:

PHP Fatal error:  Uncaught RuntimeException: Directory name must not be empty. in \v
endor\squizlabs\php_codesniffer\src\Util\Standards.php:93

Which is happening while 'check[ing] if the installed dir is actually a standard itself'.

So, might need to look into this further. In the meantime, will disable linting in the Travis build.