rename pb_query to query

I'm using R16B and I got the following error when trying to compile:

```sh
==> riemann (compile)
.../deps/riemann/src/riemann.erl:244: field pb_query undefined in record riemannmsg
.../deps/riemann/src/riemann.erl:71: Warning: opaque type riemann_event() is not exported
.../deps/riemann/src/riemann.erl:72: Warning: opaque type riemann_state() is not exported
.../deps/riemann/src/riemann.erl:242: Warning: variable 'Query' is unused
```

After looking through the protobuf definition, `pb_query` is defined as `query`. After making this change, I was able to compile and everything works.