Use latest/bio-formats5.1 redirects
Install Sphinx using pip instead of apt-get

The Travis build seems to be failing on a bug which has been fixed in
the 1.2.2 version of Sphinx that we currently use in production
Fix downloads link in sidebar to use new redirect
Merge pull request #1270 from sbesson/latest_redirect_5.1

Use latest/bio-formats5.1 redirects