B64enc and B64dec can be replaced by base64.RawURLEncoding.
Thumbprint is now implemented in go-jose, and we have the relevant version
imported already, so we can use that.
SyntaxError isn't used anywhere and can be deleted.
- Spec says the Challenge objects contain a field "Validated" not "Completed."
- The Challenge object says "Validated" should be omitempty, but wasn't a pointer.
- Swapped to using pointers so it will not be "completed":"0001-01-01T00:00:00Z"
- Sort of related to [Issue #71 in Acme-Spec](https://github.com/letsencrypt/acme-spec/issues/71)
- Remove commented-out line from Dockerfile (whoops)