VALID_URL changed to match different kinds of Tumblr-URLs

Links to Tumblr post are usually given in one of the following formats:

- http://tatianamaslanydaily.tumblr.com/post/54196191430/orphan-black-dvd-extra-behind-the-scenes
- http://tatianamaslanydaily.tumblr.com/post/54196191430

Even though the first one is prevalent, some blogs use the second one.
The changed regex matches both formats while the old one only matched the first.