Remove plonematch theme from ome-documentation repository

Sphinx theme for the HTML documentation has been moved to a standalone
repository https://github.com/openmicroscopy/sphinx_theme and is used to
generate documentation in the CI builds.

Only impact of this commit on local builds is that "-D html_theme=plonematch"
can not be used in the SPHINXOPTS envvar anymore.
Remove mention of plonematch theme in the main README
Remove plonematch html_theme from Travis build envvar
Fix  configuration section of continuous integration documentation page
Merge pull request #410 from sbesson/theme_cleanup

Remove plonematch theme from OME documentation