put blogs in series container

Further to https://github.com/guardian/frontend/pull/7264: Where a blog tag is first in the tag sequence, use this tag to populate the 'Series' container: 

Before:
![series-container-before](https://cloud.githubusercontent.com/assets/986155/5182971/053ae8c2-74a1-11e4-8c8a-b8e3babbb1a2.png)

After: 
![series-container-after](https://cloud.githubusercontent.com/assets/986155/5182974/16c11044-74a1-11e4-83ec-27b1a151ac10.png)