Avoid a south bug in hwdoc migration 0007

Turns out that south pre 0.8 had a bug when renaming columns in mysql.
That is upstream bug #1186. Monkey patching south in that specific
migration and for south versions less than 0.8 solves this successfully.
Tested on mysql 5.5 on Debian wheezy and sqlite 3.7.13

This closes #48