`Box<Fn(int) -> int + 'static>` is not accepted: "explicit lifetime bound required"

### STR

``` rust
#![feature(unboxed_closures)]

#[cfg(error)]
fn adder(y: int) -> Box<Fn(int) -> int + 'static> {
    //~^ error: explicit lifetime bound required
    box move |&: x| y + x
}

#[cfg(not(error))]
fn adder(y: int) -> Box<Fn<(int,), int> + 'static> {
    box move |&: x| y + x
}

fn main() {}
```

Version: `ebc625a` (2014-11-08)

cc @nikomatsakis 
