Moved chefspec formatter registration into the ChefRunner.  Addresses #166

This prevents the duplicate registrations.  Obviously,  I could have tested ChefRunner#initialize directly, but that seems to miss the point.  The failing case was exposed with the following type of situation:

```ruby
describe "context 1" do
  let(:chef_run) ...
  it "does what I want" do
    chef_run
  end
end

describe "context 2" do
  let(:chef_run) ...
  it "still does" do
    expect{chef_run}.to raise_error(StandardError)
  end
end
````

If the second run fails in the compile stage, the twice-registerd chefspec formatter will twice-report the information relating to the fail of the compile.