Added a missing dependency

Sunil Mohan Adaparequested to merge
harry-7:master into master

Created by: harry-7

docbook-utils package is needed to install plinth in some of the debain bases systems. So Needed to add it . Also Simple_tag functionality is not working properly in django 1.8 we need django1.9 for it So I added it as an extra dependency and steps to install it.

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