Fixes issue 237: "No such column" when renaming some columns in the migrations

Fixes issue 237 (https://github.com/rails-sqlserver/activerecord-sqlserver-adapter/issues/237) by forcing the a reload of the table_cache when renaming a column.

Why would we want to do this? Apparently, column definitions of a table are loaded once, then cached. If you want to rename an initial column, no harm is done. If you want to rename an already renamed column, or a column added at a later time, the cache doesn't know about it.

This problem is only necessary when running the migrations on SqlServer. I have not been able to reproduce it on a Sqlite or Postgres database.

I'm not sure if this is the best place/way to solve this problem, but it does the trick for me.