travis: build HTML docs

Several patches delivered incorrect restructured text as documentation.
We should be able to discover this in Travis CI.

Provide a build step for 'make htmldocs'.

Add required package graphviz.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
1 file changed