Additional check_database_schema.php check

[brendanheywood]

We should be able to run a check_database_schema.php check to confirm that all new installs and upgrades are in sync.

So the CI plugin should grab the earliest verion of install.xml, install it, then checkout the latest version on HEAD and run the upgrade script, then run check_database_schema.php and confirm it is empty

Ideally this is zero config, but maybe we need to tell travis the commit hash of the initial commit where the first install.xml file was introduced

[Peterburnett]

A patch has been submitted to include this check in PR #102