RuboCop/RSpec Upgrades

RuboCop 0.23.0 was recently released, renaming the `Rubocop` class to `Rubocop`. Also, to avoid RSpec 2.99.x deprecation warnings, I upgraded to RSpec 3.0. Any client apps/libraries seeing the following error should upgrade `aptible-tasks`:

```
rake aborted!
NameError: uninitialized constant Rubocop
```