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 though.
Merge pull request #2 from 2rs2ts/master

Fix URL output