Fix URL output

Memegenerator seem to have changed their API so now you get a fully qualified CDN URL. While we could take the approach that hubot-scripts does (see [here](https://github.com/github/hubot-scripts/blob/master/src/scripts/meme_generator.coffee#L223-L228)) it's easier just to return the new URL. Check for `https?://` to be backwards compatible, just in case.