Run Ci builds on Mysql 5.6 to test and prevent (some critical) bugs caused by the now-enabled-by-default STRICT_TRANS_TABLES setting.

To prevent such issues let's run all tests against it. Refs https://github.com/piwik/piwik/pull/279

Reused Mysql 5.6 gist provided in https://github.com/travis-ci/travis-ci/issues/1986