Bump DB version 6 to 7

And fix update script
Remove ability to migrate from version before version 6 because we drop support of SQLObject. And at one point we can't provide support for it (Python3)
Set python2 executable for update script and check if sqlo is available
Modify update script comments and text to be more comprehensive
Fix check on table exist
Merge pull request #40 from bport/sqla-bump-version

Bump DB version 6 to 7