Replace snipMate's Filename() with vim-snippets' version


It appears that vim-snippets decided to namespace their `Filename()` implementation and UltiSnips has removed their implementation completely causing the previously converted snipMate snippets to fail that use the legacy `Filename()`.

https://github.com/honza/vim-snippets/commit/80697bea6feb83c870a4fa5798e897d4561900f3
https://github.com/honza/vim-snippets/blob/master/autoload/vim_snippets.vim
https://github.com/SirVer/ultisnips/tree/master/plugin


This change goes ahead and does the `Filename()` to `vim_snippets#Filename()` in the conversion script.

The one issue w/ this is that it will place an explicit dependency for vim-snippets when using converted snippets. Which, may be undesirable. Alternatively, we could replace it with `!p fn`.

I was not completely sure the most appropriate fix for this, so if there is a better way lets do that.