Update to support rocket =0.5.0-rc.4

[kije]

First off, thanks for this great crate! :)

Rocket 0.5.0-rc.4 was just released with several breaking changes.

Relevant:

• Outcome::Failure was renamed to Outcome::Error.
• change to Cookie::build

error[E0599]: no variant or associated item named `Failure` found for enum `Outcome` in the current scope
   --> /.cargo/registry/src/index.crates.io-6f17d22bba15001f/rocket_oauth2-0.5.0-rc.2/src/lib.rs:431:33
    |
431 |                 return Outcome::Failure((
    |                                 ^^^^^^^ variant or associated item not found in `Outcome<_, (Status, _), Status>`

error[E0599]: no variant or associated item named `Failure` found for enum `Outcome` in the current scope
   --> /.cargo/registry/src/index.crates.io-6f17d22bba15001f/rocket_oauth2-0.5.0-rc.2/src/lib.rs:453:33
    |
453 |                 return Outcome::Failure((
    |                                 ^^^^^^^ variant or associated item not found in `Outcome<_, (Status, _), Status>`

error[E0599]: no variant or associated item named `Failure` found for enum `Outcome` in the current scope
   --> /.cargo/registry/src/index.crates.io-6f17d22bba15001f/rocket_oauth2-0.5.0-rc.2/src/lib.rs:478:37
    |
478 |                     return Outcome::Failure((
    |                                     ^^^^^^^ variant or associated item not found in `Outcome<_, (Status, _), Status>`

error[E0599]: no variant or associated item named `Failure` found for enum `Outcome` in the current scope
   --> /.cargo/registry/src/index.crates.io-6f17d22bba15001f/rocket_oauth2-0.5.0-rc.2/src/lib.rs:511:26
    |
511 |                 Outcome::Failure((Status::BadRequest, e))
    |                          ^^^^^^^ variant or associated item not found in `Outcome<_, (Status, _), Status>`

error[E0061]: this function takes 1 argument but 2 arguments were supplied
   --> /.cargo/registry/src/index.crates.io-6f17d22bba15001f/rocket_oauth2-0.5.0-rc.2/src/lib.rs:710:13
    |
710 |             Cookie::build(STATE_COOKIE_NAME, state)
    |             ^^^^^^^^^^^^^                  -------
    |                                            | |
    |                                            | unexpected argument of type `std::string::String`
    |                                            help: remove the extra argument
    |
note: associated function defined here
   --> /.cargo/registry/src/index.crates.io-6f17d22bba15001f/cookie-0.18.0/src/lib.rs:324:12
    |
324 |     pub fn build<C: Into<Cookie<'c>>>(base: C) -> CookieBuilder<'c> {
    |        

[jebrosen]

Thanks - I have merged #49 and released https://crates.io/crates/rocket_oauth2/0.5.0-rc.3!