update the html theme to use sphinx_holoviz_theme

[kebowen730]

@philippjfr @jlstevens I updated the conf.py file to use sphinx_holoviz_theme

[coveralls]

Coverage remained the same at 76.237% when pulling 73e7206 on update-sphinx into 08c2d44 on master.

[ceball]

I'm totally no longer competent to review this stuff, but what about https://github.com/holoviz/param/blob/master/.travis.yml#L131 - does it need to be updated too?

(Such dependencies should be in setup.py where people might have a chance of finding them...)

[ceball]

@kebowen730 Like I said, I'm not sure how this all works any more, but I can't immediately think of what else you'd need to change, so I'll merge. Could you then try pushing the appropriate tag to get a dev website re-build to happen (i.e. to update https://pyviz-dev.github.io/param) and check the build works and the site comes out ok?

I don't know if this general pattern (i.e. how to rebuild our sites) is documented somewhere. If not, it would be great to capture it somewhere.

Thanks!