Upgrade rspec

woot woooooooooot! I did it! 

I basically did everything on this page:
https://relishapp.com/rspec/docs/upgrade

Another minor nitpick: the `~>` sign in a Gemfile means: "this version or any version where only the last number is higher". So 3.0.2 would be allowed, 3.0.3, as well as 3.0.100. What would not be allowed however with this line is for example 3.1.0. Well behaved gems (and I hope that rspec-rails is one) usually promise not to break anything except on major version increases (e.g. from 2 to 3, or 3 to 4).

Now, specifying that we want anything that starts with a 3.0. only really makes sense when we have reason to assume that a higher version would break our app, which I don't think we have, so we might as well just write `~> 3.0` here.