Update ruby-sprockets to 2.3.1.

Update ruby-sprockets to 2.3.1.

**2.3.1** (February 11, 2012)

* Added bytesize to manifest
* Added Asset#bytesize alias
* Security: Check path for forbidden access after unescaping

**2.3.0** (January 16, 2012)

* Added special Sass importer that automatically tracks any `@import`ed files.

**2.2.0** (January 10, 2012)

* Added `sprockets` command line utility.
* Added rake/sprocketstask.
* Added json manifest log of compiled assets.
* Added `stub` directive that allows you to exclude files from the bundle.
* Added per environment external encoding
  (Environment#default_external_encoding). Defaults to UTF-8. Fixes issues
  where LANG is not set correctly and Rubys default external is set to ASCII.

**2.1.2** (November 20, 2011)

* Disabled If-Modified-Since server checks. Fixes some browser caching issues
  when serving the asset body only. If-None-Match caching is sufficent.

**2.1.1** (November 18, 2011)

* Fix windows absolute path check bug.

**2.1.0** (November 11, 2011)

* Directive comment lines are now turned into empty lines instead of
  removed. This way line numbers in
  CoffeeScript syntax errors are correct.
* Performance and caching bug fixes.