Clippy lint catches unseparated literal suffix

[dtolnay]

error: integer type suffix should be separated by an underscore
   --> /home/travis/build/serde-rs/json/json/src/read.rs:460:17
    |
460 |     let mut n = 0u16;
    |                 ^^^^
    |
note: lint level defined here
   --> /home/travis/build/serde-rs/json/json/src/lib.rs:120:46
    |
120 | #![cfg_attr(feature = "clippy", deny(clippy, clippy_pedantic))]
    |                                              ^^^^^^^^^^^^^^^
    = help: for further information visit https://github.com/Manishearth/rust-clippy/wiki#unseparated_literal_suffix

https://github.com/Manishearth/rust-clippy/pull/1145