Update vipers-video-quicktags.php

Vimeo videos served on a HTTPS-enabled blog will be blocked in Chrome by default:

> “The page was loaded over HTTPS, but ran insecure content…this content should also be loaded over HTTPS.”

Hence I've change this to use a [schemeless URL](http://www.paulirish.com/2010/the-protocol-relative-url/)
Merge pull request #5 from idleberg/master

Serve Vimeo videos over SSL on SSL sites