Allow URLs by chopping off http...

[nicktimko]

For the lazy (like me) who sometimes end up copying DOI URLs with the dx.doi.org prefix.

Travis-CI was choking for me, so I had to add sudo: required to the config file. Related docs.

[nicktimko]

I also don't get the failing build; when I restarted one on Travis it magically fixed itself. Here's me PRing myself with a passed build: https://github.com/nicktimko/doi2bib/pull/1