:setf FALLBACK is not available in older versions (#346)

There is no workaround for this. If we would set the filetype first via `:set
filetype=cfg` or `:setfiletype cfg`, more specialized plugins like vim-toml,
wouldn't be able to overwrite the filetype with `:setfiletype toml` anymore.

Now, we simply don't set a filetype at all for older Vim versions. Not great,
but better than an error that keeps rust.vim from getting sourced properly.

Fixes #345

ftdetect/rust.vim

@@ -1,7 +1,10 @@
 " vint: -ProhibitAutocmdWithNoGroup
 
 autocmd BufRead,BufNewFile *.rs call s:set_rust_filetype()
-autocmd BufRead,BufNewFile Cargo.toml setf FALLBACK cfg
+
+if has('patch-8.0.613')
+    autocmd BufRead,BufNewFile Cargo.toml setf FALLBACK cfg
+endif
 
 function! s:set_rust_filetype() abort
     if &filetype !=# 'rust'