Alias is_expected to are_expected

When the attribute or method being called from `its` is a plural, the spec doesn't read very well with `is_expected`.

For example:

``` ruby
describe 'something' do
  subject { :foo }
  its(:methods) { is_expected.to include :to_s }
end
```

Using `are_expected` instead of `is_expected` seems to make a little more sense here:

``` ruby
describe 'something' do
  subject { :foo }
  its(:methods) { are_expected.to include :to_s }
end
```