Issue title: Broken link release notes
 Issue body: The following link is broken in
https://github.com/opencobra/cobrapy/blob/devel/release-notes/0.6.0.md

Objectives can now easily be made quite advanced by simply crafting the right expression and assigning this as usual to model.objective, see the --> contraints and objectives notebook <--. 
http://cobrapy.readthedocs.io/en/latest/constraints_objective.html
 Comments: 
 Comment 0: Fixed
 Comment 1: Great. Thanks a lot.
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