Fix preprocess script to not break links

[trvrb]

Currently, the preprocess script will break at

https://github.com/blab/blotter/blob/master/_scripts/preprocess-markdown.rb#L69

If a link looks like:

[iPython notebook](http://github.com/trvrb/sismid/blob/master/selection/mutation-drift-selection.ipynb)

It works for the canonical

[iPython notebook](mutation-drift-selection.ipynb)

Fix the script to not break on absolute rather than relative URLs.