Fix cargo update error when compiling ws

[JohanLorenzo]

Like @isabelrios discovered this morning, cargo update breaks today's build.

/home/jlorenzo/.multirust/toolchains/nightly-2016-04-10/cargo/git/checkouts/ws-rs-012dd83a96dba3eb/d154fc5/src/handshake.rs:339:21: 339:35 error: no method named `serialize_path` found for type `&url::Url` in the current scope
/home/jlorenzo/.multirust/toolchains/nightly-2016-04-10/cargo/git/checkouts/ws-rs-012dd83a96dba3eb/d154fc5/src/handshake.rs:339                 url.serialize_path().unwrap_or("/".to_owned()),

After an investigation with @ferjm, we found out that url is now compiled in 3 different versions: 0.2.38, 0.5.9 and 1.0.0. The first 2 versions have been here so far, so the culprit is 1.0.0.

ws is not meant to be built against a version > 0.5. But it turns out only a few commits pinpoint version. d154fc5 (the revision we picked) was one of them.

r? @isabelrios

Edit: The build failed on Travis because it requires #411 to land first.

[JohanLorenzo]

Actually done in #411