Build failing on linux

I have ubuntu 16 and the terminal shows this error while building

``` sh
/Packages/JWT-2.0.1/Sources/JWT.swift:84:84: 
error: 'String' is not convertible to 'AnyObject'; 
did you mean to use 'as!' to force downcast?
  let header = encodeJSON(["typ": "JWT" as AnyObject, 
"alg": algorithm.description as AnyObject])!
```
