rust: protover.rs needs retain_hash_collection to build with older rustc versions

I was using rustc 1.19.0-nightly on master and received the following warning:

   Compiling protover v0.0.1 (file:///Users/twilsonb/tor/tor-rust/src/rust/protover)
error: use of unstable library feature 'retain_hash_collection' (see issue #36648)
   --> protover/protover.rs:259:10
    |
259 |     vers.retain(|x| !supported_versions.contains(x));
    |          ^^^^^^
    |
    = help: add #![feature(retain_hash_collection)] to the crate attributes to enable

When I added the feature declaration, everything worked fine and the tests passed.