upgrade surveyor to handle nil datetimes #4604

see changeset https://github.com/NUBIC/surveyor/compare/v1.4.0...98225cd
(98225cd is master at this time)

(copied from eefb639fb2cf6ef4ef9761ea01bb438b9576dc8d)
Merge pull request #285 from NUBIC/bug_4604_nil_datetime_breaks_surveyor

Bug 4604 nil datetime breaks surveyor