ethcore-util build fails on Ubuntu

```
Compiling ethcore-util v1.7.0 (https://github.com/paritytech/parity#125aa0ae)
error: use of unstable library feature 'retain_hash_collection' (see issue #36648)
--> /home/ryepdx/.cargo/git/checkouts/parity-dc9825eb65b3adf1/125aa0a/util/src/memorydb.rs:106:13
|
106 | self.data.retain(|_, &mut (_, rc)| rc != 0);
| ^^^^^^

error: aborting due to previous error

error: Could not compile `ethcore-util`.
Build failed, waiting for other jobs to finish...
error: build failed
Makefile:5: recipe for target 'all' failed
make: *** [all] Error 101
make: Leaving directory '/home/ryepdx/Code/ethrun'
```