Resolving DOI resolution URI

With the update to hydra-remote_identifier v0.6.4, the remote URL
resolver is configurable, but defaults to http://dx.doi.org/.

See release notes for hydra-remote_identifier:
https://github.com/projecthydra-labs/hydra-remote_identifier/releases/tag/v0.6.4
Merge pull request #92 from ndlib/resolving-doi-resolution-url

Resolving DOI resolution URI