Support for lazy loading UltiSnips

I use [vim-plug](https://github.com/junegunn/vim-plug) to lazy load UltiSnips on specific filetypes. When lazy loaded, UltiSnips doesn't appear in the `runtimepath`, and vim-go subsequently doesn't set `g:UltiSnipsSnippetDirectories`. It will not harm to set this variable anyway, regardless of Ultisnips is installed or not.