Change url to image-url in scss file

Per:

http://guides.rubyonrails.org/asset_pipeline.html

when including an asset via an Asset Pipeline
managed file, you can no longer use the `url()`
method, you must use `image-url` so that the Asset
pipeline properly changes the directory to search
for the asset.
Merge pull request #75 from tiger-swallowtails-2014/correct-path-to-welcome-jam

Merging myself as per Steven.