Set critical CSS cache entries to never expire.

Author: taranda

README.md

@@ -105,6 +105,8 @@ In addition to the `critical_path_css:generate` rake task described above, you a
 rake critical_path_css:clear_all
 ```
 
+NOTE: The `clear_all` and `clear_matched` methods will not work with Memcached due to the latter's incompatibility with Rails' `delete_matched` method.  We recommend using an alternative cache such as [Redis](https://github.com/redis-store/redis-rails).
+
 Careful use of these methods allows the developer to generate critical path CSS dynamically within the app.  The user should strongly consider using a [background job](http://edgeguides.rubyonrails.org/active_job_basics.html) when generating CSS in order to avoid tying up a rails thread.  The `generate` method will send a GET request to your server which could cause infinite recursion if the developer is not careful.
 
 A user can use these methods to [dynamically generate critical path CSS](https://gist.github.com/taranda/1597e97ccf24c978b59aef9249666c77) without using the `rake critical_path_css:generate` rake task and without hardcoding the application's routes into `config/critical_path_css.yml`.  See [this Gist](https://gist.github.com/taranda/1597e97ccf24c978b59aef9249666c77) for an example of such an implementation.

lib/critical-path-css-rails.rb

@@ -4,7 +4,8 @@ module CriticalPathCss
   CACHE_NAMESPACE = 'critical-path-css'
 
   def self.generate(route)
-      Rails.cache.write(route, CssFetcher.new.fetch_route(route), namespace: CACHE_NAMESPACE)
+      Rails.cache.write(route, CssFetcher.new.fetch_route(route),
+                        namespace: CACHE_NAMESPACE, expires_in: nil)
   end
 
   def self.generate_all