Squash all mailboxer migrations into a single one

Mailboxer user doesn't need to know about internal details of mailboxer
development history. This task was half implemented in commit
1901023668d087b0bd4ca498e63e5c1c0779e001, but for some reason only some
migrations were merged into the first one. Merging this would also close
PR #140.
Merge pull request #204 from deivid-rodriguez/squash_migrations

Squash all mailboxer migrations into a single one