Fix foreign key creation

Migrations don't work with last master because after namespacing
mailboxer (6e7e042cc8c44958688c686d95f84373a42519e7), column to be used
by the foreign key is not correctly guessed anymore. This was not
detected by the test suite because it uses sqlite and foreign keys are
probably ignored by default in sqlite. It's failing in postgresql
though. This commit fixes it by manually expliciting the column.
Remove duplicated migration

This migration was "merged" into main migration.
(20110511145103_create_mailboxer.rb) in commit
6e7e042cc8c44958688c686d95f84373a42519e7, so it now fails with a
duplicate column error. Test suite is not failing because the migration
was correctly removed from the test suite in that commit.

This makes me think that you shouldn't probably maintain two sets of
duplicated files in the repo, because bugs like this one are more likely
to be introduced.
Merge pull request #203 from deivid-rodriguez/fix_migrations

Fix migrations