Return chained image_tag to fix "size" attribute handling.

A call was originally being made to image_tag_without_retina, but the value was discarded and then it returned tag("img") which does not have the same effect. This changes the return to be the actual call to image_tag_without_retina.
Merge pull request #14 from scienceexchange/master

Return chained image_tag to fix "size" attribute handling.