Add MySQL 5.6 to test matrix

MySQL 5.6.10 represents the first GA release of 5.6. However, it cannot be used with Stash due to a serious issue with its query optimizer.

This issue has 2 deliverables:

• Monitor the MySQL bug for a fix version and validate it
• Deploy an image with the fixed version to the build farm and add it to the DAO matrix tests to validate 5.6 support going forward

Until a version including a fix for that issue is specified, 5.6 cannot be used by Stash, and will not be supported if deployed at customer sites. Stash 2.2 includes changes, made under STASH-3133, which will block attempts to start Stash when connected to MySQL 5.6, as well as to block attempts to migrate from HSQL to MySQL 5.6 or setup MySQL 5.6 as the external database.

Customers interested in using MySQL 5.6 with Stash may track this issue for status, as well as the referenced MySQL bug.

Note: This bug also precludes the use of Amazon Aurora, which as of 2017-03-31 is based on MySQL 5.6.10a