country_select 3.x support

[hantuzun]

subdivision_select is depending on country_select 2.x.

Here's the error when country_select and subdivision_select are added to Gemfile together.

Bundler could not find compatible versions for gem "country_select":
  In snapshot (Gemfile.lock):
    country_select (= 3.1.1)

  In Gemfile:
    country_select

    subdivision_select was resolved to 0.0.6, which depends on
      country_select (~> 2.0)

country_select 3.x support could be required for some.