Update unite.vim config for plugin api changes

I looked through [unite.vim commits](https://github.com/Shougo/unite.vim/commits/master) for a bit to see where this changed, but got bored. This PR removes the following errors when starting vim:

```
~/projects/foo /master> vim README.markdown
[unite.vim] You cannot set "ignorecase". Please set "context.ignorecase" by unite#custom#profile() instead.
[unite.vim] You cannot set "ignorecase". Please set "context.ignorecase" by unite#custom#profile() instead.
[unite.vim] You cannot set "ignorecase". Please set "context.ignorecase" by unite#custom#profile() instead.
Press ENTER or type command to continue
```

The errors are coming from https://github.com/Shougo/unite.vim/blob/1c8d3cb70f33766fb844fac10031902ce3ab1f2a/autoload/unite/custom.vim#L72 in unite.vim.

Technically, I'm only *assuming* this preserves the desired configured behavior. :trollface: