Remove Debian "Sid" (aka unstable) from docs

We are no longer building packages for sid/unstable. This is because
these package builds will often fail, and block successful build for
other platforms that we support. Additionally, this OS target is of
questionable value, given we already build against Debian testing.
Merge pull request #339 from melissaanne/master

Remove Debian "Sid" (aka unstable) from docs