Correct erroneous output from method rebuild_shortcode in class GistPress

Replace $rawattr with $attrs in method rebuild_shortcode in
class GistPress as per @bradyvercher's comment in
[Issue 34](https://github.com/bradyvercher/gistpress/issues/34)

Old Erroneous Behavior: [gist 7243951 1]
New Corrected Behavior: [gist id="7243951"]