remove deprecation warnings when we using rails 5
remove deprecation warnings when we using rails 5

DEPRECATION WARNING: `render :text` is deprecated because it does not
actually render a `text/plain` response. Switch to `render plain: 'plain
text'` to render as `text/plain`, `render html: '<strong>HTML</strong>'`
to render as `text/html`, or `render body: 'raw'` to match the
deprecated behavior and render with the default Content-Type, which is
`text/html`.

cnosuke/blank_image