Add gemspec license. Update sprockets dependency.
Add gemspec license. Update sprockets dependency.

zhelezko/jekyll-assets,jekyll/jekyll-assets,jekyll/jekyll-assets,zhelezko/jekyll-assets,jeremy/jekyll-assets,jeremy/jekyll-assets,jekyll/jekyll-assets,jekyll-assets/jekyll-assets,jeremy/jekyll-assets,jekyll/jekyll-assets,jekyll-assets/jekyll-assets,mattbeedle/jekyll-assets,mattbeedle/jekyll-assets,mattbeedle/jekyll-assets,zhelezko/jekyll-assets,jeremy/jekyll-assets,mattbeedle/jekyll-assets,jekyll-assets/jekyll-assets