Quick hack for #324

As reported by #324, this code doesn't work since v0.14.0.

```ruby
describe 'sshd daemon' do
  it 'has a running service of ssh' do
    expect(service('ssh')).to be_running
  end
end
```

It seems that ``expect`` notation does not call custom matchers.

One-liner ``should`` notation works fine.
Merge pull request #325 from serverspec/expect-does-not-call-custom-matchers

Quick hack for #324