Recognise all even minor versions as stable

The old pattern for determining stable versions only matches if the 2nd
part is exactly two, four, six or eight. Change it to be ready for a
possible 1.0 or 0.10 stable version in the future.
v0.3.1
zsh working maybe
v0.4.0
`nave clean` more thoroughly

Packaging node binaries per platform slightly broke clean-up.

"*-linux-x86.tgz" files were left in my src dir after `nave clean`.
v0.4.1
Don't trigger infinite reloading in oh-my-zsh
v0.4.2
fix oh-my-zsh update behaviour
v0.4.3
Check for the mandatory `version` parameter in `nave install`
v0.4.4