Replace tinyurl with non-commercial URL shortener

[msporny]

@VladimirAlexiev reported this issue in digitalbazaar/jsonld.js#549 :

TinyURL has gone commercial:

Have 14 days of a TinyURL Subscription on us!
We want to treat you to a free trial of TinyURL's paid features. Just click the button below to sign up and avail of features like:
Once you’ve signed up, you have 14 whole days to take your link management and branding to the next level.

Please replace it with some other shortener service.