Fix Travis CI builds 

* fix integration tests by MySQL5.6 pre-install (thanks @kandy for proper code snippet)
* add PHP 5.6 variation
* remove unused phpcs installation via PEAR

After that only three errors caused by one reason left in integration tests:
```
1) Magento\Sales\Controller\Adminhtml\Order\CreateTest::testLoadBlockActionData
Zend_Cache_Exception: Datas must be string or set automatic_serialization = true
2) Magento\Sales\Controller\Adminhtml\Order\CreateTest::testIndexAction
Zend_Cache_Exception: Datas must be string or set automatic_serialization = true
3) Magento\Sales\Controller\Adminhtml\OrderTest::testAddressActionNoVAT
Zend_Cache_Exception: Datas must be string or set automatic_serialization = true
```
(identical for [PHP 5.5](https://travis-ci.org/orlangur/magento2/jobs/43276708) and [PHP 5.6](https://travis-ci.org/orlangur/magento2/jobs/43276711), no error on [PHP 5.4](https://travis-ci.org/orlangur/magento2/jobs/43276705))