Replace spread operator with Object.assign for ES2015 compatibility

The babel-preset-es2015 used by this project doesn't support
object spread syntax, which was added in ES2018. Using Object.assign()
instead maintains the same functionality while being compatible with
the existing Babel configuration.

Author: Arpith Siromoney

src/lib/feedUtils.js

@@ -59,7 +59,7 @@ function buildArticleKey(hash) {
  * @returns {Object} Article with hash, score, and feedurl added
  */
 function processArticle(article, feedURI, hashFn, scoreFn) {
-  const processed = { ...article };
+  const processed = Object.assign({}, article);
   processed.hash = hashFn(article);
   processed.score = scoreFn(article);
   processed.feedurl = feedURI;