Update NixOS pages

[Chris Warburton]

Noticed a few out of date things on the NixOS project pages. It would be
nice to bump these a bit closer to the present, since they come up in
Google results, etc.

I've noticed that in "continuous integration" we use 'readFile' to get
data into Nix at eval time. It would be better to use 'import'.

Also, the description of 'latestGit' could do with being updated. For
example, including the 'stable' argument would be nice.