Fails to generate JUnit schema

Schema source: [link](https://raw.githubusercontent.com/windyroad/JUnit-Schema/master/JUnit.xsd)

Output:
```
$ ./target/release/gen JUnit.xsd
// Input: "JUnit.xsd"
thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/libcore/option.rs:345:21
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

```