Uncomment and fix URL tests

[bannanc]

I wrote tests for urls in the documentation, following the methods used by openforcefield. Unfortunately, a couple of the urls that work when I click on them from READMEs were failing in these tests. I decided it wasn't worth my time to fight with since they are working links.

I'm not convinced we want our tests to fail every time someone else's server goes down so I don't know if having these tests are worth it in the first place.