Fix Trave CI builds: "error: augeas.h: No such file or directory"

Created by: SunilMohanAdapa

Plinth has integration with Travis CI such that every pull request is automatically tested with it. Currently, it is failing due to a missing dependency on augeas development libraries.

https://travis-ci.org/freedombox/Plinth/jobs/215599849

Fix this by:

• Installing augeas development libraries or by (preferred)
• Installing python-augeas package so that PIP doesn't install it.