Append 'unsubscribe’, 'resubscribe' function's return-type annotation

For fix this compile error.
```
> tsc --noImplicitAny arbiter/Arbiter.d.ts
Arbiter.d.ts(124,9): error TS7010: 'unsubscribe', which lacks return-type annotation, implicitly has an 'any' return type.
Arbiter.d.ts(129,9): error TS7010: 'resubscribe', which lacks return-type annotation, implicitly has an 'any' return type.
```