Update ruby-sprockets to 2.8.3. Fix security problem of CVE-2014-7819.
**2.8.2** (December 10, 2012)
* Fixed top level Sass constant references
* Fixed manifest logger when environment is disabled
**2.8.1** (October 31, 2012)
* Fixed Sass importer bug
**2.8.0** (October 16, 2012)
* Allow manifest location to be seperated from output directory
* Pass logical path and absolute path to each_logical_path iterator
**2.7.0** (October 10, 2012)
* Added --css-compressor and --js-compressor command line flags
* Added css/js compressor shorthand
* Change default manifest.json filename to be a randomized manifest-16HEXBYTES.json
* Allow nil environment to be passed to manifest
* Allow manifest instance to be set on rake task
**2.6.0** (September 19, 2012)
* Added bower component.json require support
**2.5.0** (September 4, 2012)
* Fixed Ruby 2.0 RegExp warning
* Provide stubbed implementation of context *_path helpers
* Add SassCompressor
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