Vim: Update PHP setter snippet to recognise 'self' as a return value.

If not specified, the function will be assumed to return void or a
different value.
Vim: Shut up if Python is not available for snippets.
Vim: +mbbill/undotree, -sjl/gundo.vim.

Gunde is better as it has live diff preview, but Undotree is native so
no Python required.