Move migration into a transaction; #141
Move migration into a transaction; #141

This dodges a race condition where we could fail to log some goal
changes.

eXcomm/gratipay.com,studio666/gratipay.com,studio666/gratipay.com,bountysource/www.gittip.com,bountysource/www.gittip.com,bountysource/www.gittip.com,mccolgst/www.gittip.com,eXcomm/gratipay.com,studio666/gratipay.com,eXcomm/gratipay.com,studio666/gratipay.com,mccolgst/www.gittip.com,bountysource/www.gittip.com,gratipay/gratipay.com,mccolgst/www.gittip.com,eXcomm/gratipay.com,mccolgst/www.gittip.com,gratipay/gratipay.com,gratipay/gratipay.com,gratipay/gratipay.com