Revert "GSoC blog post: updated link to John Detlef blog"

@jdetle changed his site structure again so now the blog posts appear again
directly at http://jdetle.com

This reverts commit 4be50b2.

_posts/2016-04-26-gsoc.md

@@ -71,7 +71,7 @@ summer. We plan to have all discussions public on the
 
 [pca]: https://en.wikipedia.org/wiki/Principal_component_analysis
 [dm]: https://en.wikipedia.org/wiki/Diffusion_map
-[john-blog]: http://www.jdetle.com/blog.html
+[john-blog]: http://www.jdetle.com
 [fiona-blog]: http://fiona-naughton.github.io/blog/
 [devel]: {{site.author.email}}
 [us]: https://en.wikipedia.org/wiki/Umbrella_sampling