Gem file upgrades

[tansaku]

@armandofox I've tested the assignment and everything works with the latest version of rspec (3.5) and activemodel (5.0) ...

[armandofox]

I think this should be OK. as we discovered this week, rspec-mocks >= 3.5 requires mocked methods to exist by default, but i don't think that affects this project. if you wanted to be super paranoid you could check the Rspec version in the RSpec::Configure.do |config| block and turn off verified doubles just in case, but pretty sure this assignment doesn't use them. If there is a place we're still tracking Textbook issues, though, may be a good idea to add this since the book is written assuming that mocking nonexistent methods will work