Skip documentation update if CI build is invoked not on master branch.

Also:
- Setup doxygen configuration on the fly to use minimal theme and to produce SVG instead of PNG.
- Change how the GH_TOKEN is supplied as the GIT version in Travis VM (Ubuntu) does not seem to read the credentials from file store correctly.
- Only generate site documentation after the CI build test has passed.
- Add Ubuntu PPA to install a more recent version of Doxygen.