doc(download/index.md): update Ubuntu/OSX versions 
doc(download/index.md): update Ubuntu/OSX versions 

 - Ubuntu 15.04 is not supported (out of the life cycle). Ubuntu 15.10 support is added.
 - We prepare pre-compiled binaries for OSX 10.10 and OSX 10.11.

[skip ci]
leanprover/leanprover.github.io,avigad/leanprover.github.io,avigad/leanprover.github.io,leanprover/leanprover.github.io,leanprover/leanprover.github.io,avigad/leanprover.github.io