Fix `&runtimepath` list insertion index
Fix `&runtimepath` list insertion index

This corrects the intended behavior of inserting dein's runtime path
immediately prior to `$VIMRUNTIME`, and addresses the edge case where
`$VIMRUNTIME` is the first path in `&runtimepath`.

Shougo/dein.vim,Shougo/dein.vim,Shougo/dein.vim