PR https://github.com/woocommerce/woocommerce/pull/23082 made some changes to the Travis configuration and one of those changes broke the PHPCS when running inside Travis with the following error:
```
ERROR: Referenced sniff "WooCommerce-Core" does not exist
```
For example: https://travis-ci.org/woocommerce/woocommerce/jobs/509471213#L659
This problem went unnoticed during the PR review as no PHP file was modified in it and thus there was no file for PHPCS to check.
This commit fixes the error above by installing the Composer package `woocommerce/woocommerce-sniffs` which is the package that provides `WooCommerce-Core` sniffs and which installs `wp-coding-standards/wpcs` as one of its requirements. I couldn't find an easy way to make this work installing `woocommerce/woocommerce-sniffs` globally so that is why this commit also removes the `global` parameter when calling composer.
This commit renames PHPCS configutarion file to `phpcs.xml`. With the default name, it is not necessary to use the argument `--standard=phpcs.ruleset.xml` anymore when calling `phpcs`.
PR #17680 added a new PHP 7.1 Travis build job to generate code coverage report. PHPCS was configured to run on all PHP 7.1 build jobs. So this means that after #17680 was merged, Travis started running PHPCS twice.
This commit fixes this issue by setting a new environment variable called `$RUN_PHPCS` and using this variable, instead of the PHP version, to decide when to run PHPCS.
Rodrigo Primo
Gerhard
Mike Jolley
Claudio Sanches