Reveals another MacOS build breakage in the Core Text shaper.

Trusty has gtk-doc 1.20 which has better markdown support which should
fix some of the brokenness in the newly added documentation. The build
seems to be as fast as the current container-based build on Precise.

Fixes failures in others' forks, like:
https://travis-ci.org/khaledhosny/harfbuzz/jobs/99719710#L3912

To lower the number of gtk-doc warnings.

I've set up a secure var for behdad/harfbuzz through Travis web interface, no need
to set the var here.

Build docs in Travis and push them to the gh-pages branch, which makes
them available at http://behdad.github.io/harfbuzz/

Fixes https://github.com/behdad/harfbuzz/issues/182

I never could get it to work, and it's a proprietary platform.

This essentially disables coverity-scan right now, until we find
a pattern to continuously submit branches there.

For background reasoning, see:
Fixes https://github.com/behdad/harfbuzz/issues/171

First try...

Apparently the version on Travis is 1.32.0, while we require 1.34.0.

We want default gcc / clang warnings to fail the CI build.

Initial setup of gtk-doc. Straight forward setup following the gtk-doc
instructions. Ignore some troublesome types in src/hb-gobject.h. To
build use "./autogen.sh --enable-gtk-doc" then "make". Docs are in
harfbuzz/docs/reference/html/index.html.

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