Delink headers above prev/next

The links to previous and next blog posts looked jarring to me as a 2-line collective link.

Before:

![before](https://cloud.githubusercontent.com/assets/4592/5210903/a0f13da4-75a3-11e4-92df-a5d85f38549a.png)

After:

![after](https://cloud.githubusercontent.com/assets/4592/5210904/a2b6091c-75a3-11e4-816e-083220d5b841.png)

Fixes #315.