Extern crate warnings in test suite

[dtolnay]

As of rustc 1.21.0-nightly (e26688824 2017-08-27), cargo test gives the following warnings.

   Compiling serde_json v1.0.2
warning: unused extern crate
   --> src/lib.rs:323:1
    |
323 | extern crate core;
    | ^^^^^^^^^^^^^^^^^^
    |
    = note: #[warn(unused_extern_crates)] on by default

warning: unused extern crate
  --> tests/stream.rs:11:1
   |
11 | extern crate serde;
   | ^^^^^^^^^^^^^^^^^^^
   |
   = note: #[warn(unused_extern_crates)] on by default

[dtolnay]

The extern crate core is no longer present, and the extern crate serde was fixed by rust-lang/rust#44825.