Upgrade activerecord... a little bit.

activerecord 4.0.1 requires minitest 4.x, and all of the exercism tests
are minitest 5.x. I can't win. Either activerecord is up-to-date, or
minitest is. No matter what, my gem dependencies will be reported as
outdated.

I suppose the bottom line is that I care more about minitest than
activerecord, so activerecord can go languish in its miserable
outdatedness.