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

It appears that vim-snippets decided to name space their Filename() implementation and UltiSnips has removed their implementation completely causing the previously converted snipMate snippets to fail that use the legacy Filename().

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.