Update to rayon 1.0

[Boscop]

When I try to use mozjpeg-sys = "0.5.11" in the same project as jpeg-decoder = "0.1.14" I get:

    Updating registry `https://github.com/rust-lang/crates.io-index`
error: failed to select a version for `rayon-core` (required by `rayon`):
all possible versions conflict with previously selected versions of `rayon-core`
  version 1.3.0 in use by rayon-core v1.3.0
  possible versions to select: 1.4.0

Because jpeg-decoder depends on rayon 1.0 but mozjpeg-sys's build.rs depends on nasm-rs which depends on rayon 0.9

[Gavin Massey]

I merged #18. Which should fix the problem