Revert "[vim] Use goreturns as go fmt command"

This reverts commit 4cbe6c0.

`goreturns` was returning output to stdout, rather than writing to a
file. I was hoping for a drop-in replacement for `gofmt`/`goimports`.

See also:
fatih/vim-go#208

vimrc

@@ -57,7 +57,7 @@ Plugin 'tpope/vim-fugitive.git'
 Plugin 'tpope/vim-rails.git'
 
 "Golang settings for vim-go plugin
-let g:go_fmt_command = "goreturns"
+let g:go_fmt_command = "goimports"
 let g:go_highlight_functions = 1
 let g:go_highlight_methods = 1
 let g:go_highlight_structs = 1