Issue title: Breadcrumbs fail to output prominent keyword
 Issue body: 
* [ ] I've read and understood the [contribution guidelines](https://github.com/Yoast/wordpress-seo/blob/trunk/.github/CONTRIBUTING.md).
* [ ] I've searched for any related issues and avoided creating a duplicate issue.

### Please give us a description of what happened.

Select prominent keywords in the SEO-Advanced-Breadcrumb drop down box for posts

### Please describe what you expected to happen and why.

Prominent keywords for the post would then appear in the breadcrumb pathway. They did not


### How can we reproduce this behavior?
1. Go to SEO-Advanced-Breadcrumb drop down box for posts and select Prominent Word

![screen shot 2017-08-23 at 12 00 04 pm](https://user-images.githubusercontent.com/8238074/29625756-9ffb6084-87fa-11e7-8e14-04427480468e.png)


2.See prominent keywords appear on post

![screen shot 2017-08-23 at 12 00 29 pm](https://user-images.githubusercontent.com/8238074/29625772-aad9c1d0-87fa-11e7-8c53-a82125b742c4.png)

3.See that prominent words fail to appear in the breadcrumb pathway

![screen shot 2017-08-23 at 12 00 50 pm](https://user-images.githubusercontent.com/8238074/29625807-bfeb1e48-87fa-11e7-9196-0a23464f61de.png)


### Technical info
* WordPress version: 4.8.1
* Yoast SEO version: 5.3.2
* Relevant plugins in case of a bug:

 Comments: 
 Comment 0: Please inform the customer of conversation # 216790 when this conversation has been closed.
 Comment 1: Can you move this to the premium repository, because prominent words is a premium feature?
 Comment 2: related: https://github.com/Yoast/wordpress-seo-premium/issues/1326

@andizer  Done
{'source': 'GithubIssues', 'repo_name_with_owner': 'Yoast/wordpress-seo', 'issue_id': 252334734, 'repo_stars': 1707.0}