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 noticed a couple other issues which I have added fixes for:
- Using the HTTPS version of rubygems so that bundler will not give warnings anymore
- Removed 1.8.7 from `.travis.yml` since it has never worked for this gem
- Added a new rake task called `specs:travis` that runs all the tests, minus 2 specific tests that work locally, but not on travis.  I tagged these two tests with `travis_error => true`.  `specs:all` still works the same.