Add Guides note to change cache name space as Entry data is incompatible between 3.2 and 4.0
[ci skip] update rails version to 3.2.21
Mounted Rack apps should have default named routes based on app name

This fixes a regression in 4.2.0 from 4.1.8.

https://github.com/rails/rails/pull/17823 fixed a similar regression regarding _explicitly_ named routes for a mounted Rack app, but there was another regression for the default value.

With a route like:

    Rails.application.routes.draw do
      mount Mountable::Web, at: 'some_route'
    end

The "Prefix" column of rake routes gives the following:

- 4.1.8:         mountable_web
- 4.2.0.beta1-4: [nothing]
- 4.2.0.rc1:     [nothing]
- 4.2.0.rc2:     some_route   <- regression

This fixes the default to go back to being based off the name of the class like the docs specify: https://github.com/rails/rails/blob/785d04e3109f69d0b9b9f4732179592f0ef04e52/actionpack/lib/action_dispatch/routing/mapper.rb#L558-L560

Explicitly named routes still work correctly per https://github.com/rails/rails/pull/17823:

    Rails.application.routes.draw do
      mount Mountable::Web, at: 'some_route', as: 'named'
    end

- 4.1.8:         named
- 4.2.0.beta1-4: [nothing]
- 4.2.0.rc1:     [nothing]
- 4.2.0.rc2:     named
Merge pull request #17943 from jeremywadsack/doc_cache_importability

Add Guides note to change cache namespace
Merge pull request #17942 from yui-knk/fix/upgrading.md

[ci skip] update rails version to 3.2.21
Merge pull request #17944 from tjschuck/mounted_named_routes_regression

Mounted Rack apps should have default named routes based on app name
[ci skip] add `enable_extension` to PostgreSQL hstore example
Merge pull request #17950 from y-yagi/postgres_guide

[ci skip] add `enable_extension` to PostgreSQL hstore example