Don't complain about undefined function

When lazy loading UltiSnips on specific filetypes ([vim-plug](https://github.com/junegunn/vim-plug) and [NeoBundle](https://github.com/Shougo/neobundle.vim) can do that), UltiSnips#FileTypeChanged() function can be undefined while ftdetect/UltiSnips.vim is loading.
Merge pull request #323 from opennota/master

Check for existing of FT function before calling it.