Update gemspec to use rspec 3

[toothfairy]

Rspec 2 is old and bunk, Rspec 3 is new and cool

[sindrenm]

This is true! However, there are some backwards-incompatible things to handle. 😃

[toothfairy]

I hope we don't have any of them here, cause I already spent like 3 hours replacing should to expect syntax this week

[timkurvers]

Expect-syntax is supported in RSpec 2, too, no?

[toothfairy]

it is! but should syntax is deprecated in rspec 3