[TravisCI] Patch `rake db:fixtures:dump` task to work in Travis

- Fix ActiveRecord connection to use Rails.env, as described in the
  rake task description, instead of being hard-coded to :development.
  Travis apparently sets RAILS_ENV to 'test', so db:migrate loads up the
  test database. But db:fixtures:dump was hardcoded to development,
  hence causing it to fail.

- Fix skipped tables from 'schema_info' to 'schema_migrations'. This was
  changed in Rails since 2.1:
  http://api.rubyonrails.org/classes/ActiveRecord/Migration.html

- Ensure that test/fixtures directory exists before dumping.

In general fixtures are a pain to manage, especially when the project
gets bigger and test coverage increases. It's much better to ditch them
altogether and use a replacement like factory_girl or machinist.