Add Shougo/neosnippet-snippets. Fixes #539.

[Shougo/neosnippet.vim](https://github.com/Shougo/neosnippet.vim) version 4.1 split the repository into two and the other half is now missing. This causes following errors on Mac and Cygwin at least:
```
neosnippet default snippets cannot be loaded.
You must install neosnippet-snippets or disable runtime snippets.
```
I'm not sure why gVim (on Windows) doesn't warn about this.

This PR adds Shougo/neosnippet-snippets as we're already using [the "extra" snippets](https://github.com/honza/vim-snippets), so it makes sense to include the default snippets instead of disabling them. 

More info of the breaking change: https://github.com/Shougo/neosnippet.vim/commit/5dba6a034ed85fc6105f7aa052830cc39ad262bf