Fork jsonwebtoken until Sergio's patch lands

(Or something like it).

Rocket and jsonwebtoken depend on different versions of `cookie`, and those two
versions depend on different versions of `ring`, and `ring` has a policy of not
being linked more than once into a binary. You'll get an error like this:

```
error: multiple packages link to native library `ring-asm`, but a native library can be linked only once

package `ring v0.11.0`
    ... which is depended on by `cookie v0.9.2`
    ... which is depended on by `rocket v0.3.9`
    ... which is depended on by `changes v0.1.0 (file:///home/coleman/Code/Rust/changes)`
links to native library `ring-asm`

package `ring v0.12.1`
    ... which is depended on by `jsonwebtoken v4.0.1`
    ... which is depended on by `changes v0.1.0 (file:///home/coleman/Code/Rust/changes)`
also links to native library `ring-asm`
```

My workaround: fork `jsonwebtoken` and depend on an older `ring` that Rocket
transitively depends on, v0.11.0

Refs:
briansmith/ring#619
briansmith/ring#535

Cargo.toml

@@ -14,7 +14,7 @@ error-chain = { version = "0.11", default-features = false }
 serde_json = "1.0"
 serde_derive = "1.0"
 serde = "1.0"
-ring = { version = "0.12.0", features = ["rsa_signing", "dev_urandom_fallback"] }
+ring = { version = "0.11.0", features = ["rsa_signing", "dev_urandom_fallback"] }
 base64 = "0.9"
 untrusted = "0.5"
 chrono = "0.4"