Fix neovim errors

From latest OSX update, some plugins started to not working. Especially, deoplete and UltiSnips show errors.

The reason may be missing Python3, so the following commands will fix them:

brew install neovim pip3 install -U neovim nvim # in neovim :UpdateRemotePlugins 

If you still have troubles, run checkhealth in vim.

:checkhealth