Description:
    Stubs out a new database migration. Pass the migration name, either
    CamelCased or under_scored, as an argument. A migration class is generated
    in db/migrate prefixed by the latest migration number.

Example:
    `./script/generate migration AddSslFlag`

    With 4 existing migrations, this creates the AddSslFlag migration in
    db/migrate/005_add_ssl_flag.rb