Use `Thread.current` for thread-safeness for now

  * This should probably be changed to use
    [RequestStore](https://github.com/steveklabnik/request_store) since
    I can imagine that `country_select` is used across so many servers
  * I removed `CountrySelect.locale=` because that should be handled
    with `I18n.locale`. I have, however, kept the getter and it just
    returns `I18n.locale`. This seemed the right choice from a
    testing/API-completeness perspective.
Remove unused `DEFAULTS`
Add seeded default hash back. :derp:
Triage Travis CI configuration

  * Use latest rubies for travis
  * Switch to travis-handled rbx version; add dependent gems
  * Use latest appraisal gem that supports Bundler platforms
  * Update appraisal gemfiles to use latest appraisal
  * Add fragile rubies to allowable failures in travis-ci
Merge pull request #1 from scudco/auzroz-localization

Fixup some of the Thread safety issues