Pins JWT to ~> 1.0.0 and fixes tests.

JWT started to return both the payload and a header when decoding a token since
[this pull request](https://github.com/progrium/ruby-jwt/pull/35). This has
since become part of the version 1.0 release, so this gem might as well
encourage usage of 1.0 and get fixed tests for that.
Merge pull request #97 from philnash/jwt-test-failures

Pin JWT to ~> 1.0.0 and fix tests