Remove relative segments when prepending

Consistent with prepending behavior prior to ember-cli#35. Which even removes all depths of relative segments, regardless if it resolves to a valid asset. Perhaps that was never intentional, but nobody's complained.

Fixes ember-cli#36.

index.js

@@ -119,8 +119,8 @@ AssetRewrite.prototype.rewriteAssetPath = function (string, assetPath, replaceme
     replaceString = match[1].replace(assetPath, replacementPath);
 
     if (this.prepend && replaceString.indexOf(this.prepend) !== 0) {
-      var removeLeadingSlashRegex = new RegExp('^/?(.*)$');
-      replaceString = this.prepend + removeLeadingSlashRegex.exec(replaceString)[1];
+      var removeLeadingRelativeOrSlashRegex = new RegExp('^(\\.*/)*(.*)$');
+      replaceString = this.prepend + removeLeadingRelativeOrSlashRegex.exec(replaceString)[2];
     }
 
     newString = newString.replace(new RegExp(escapeRegExp(match[1]), 'g'), replaceString);

tests/fixtures/relative-urls-prepend/input/assets/url-in-styles.css

@@ -1,3 +1,3 @@
 .sample-img{width:50px;height:50px;background-image:url('images/foobar.png')}
 
-.sample-img2{width:50px;height:50px;background-image:url('../img/saturation.png')}
+.sample-img2{width:50px;height:50px;background-image:url('../../../img/saturation.png')}