Allow .standalone_migrations to override partial configuration

If I have a `.standalone_migrations` file that looks like this:

    db:
        seeds: db/seeds.rb
        migrate: db/migrate
    config:
        database: db/config.yml

Then the `schema` value will be `nil`.  The `schema` value should be the default value and not `nil`.

This pull request updates the configurator to work like this.  I have also updated the tests to ensure that the default values will be used.

I also updated rubygems to use the HTTPS version to get rid of the warning message that bundler gives when running bundle.