vsn field added to hello.app.src

Hello,

I added `vsn` field to hello.app.src, without it when we try to get hello with rebar error:

```
ERROR: Failed to get app value 'vsn' from '/home/user/ChicagoBoss/deps/hello/src/hello.app.src'
ERROR: 'get-deps' failed while processing /home/user/ChicagoBoss/deps/boss_db: rebar_abort
```