Added 'nowarn_deprecated_type' to rebar configuration.

When compiling `poolboy` with Erlang/ OTP 17, the compiler rises some warnings/errors saying the following:

```shell
~/poolboy/src/poolboy.erl:16: type queue/0 is deprecated and will be removed in OTP 18.0; use use queue:queue/0 or preferably queue:queue/2
~/poolboy/src/poolboy.erl:17: type queue/0 is deprecated and will be removed in OTP 18.0; use use queue:queue/0 or preferably queue:queue/2
```

I couldn't compile `boss_db` for R17 because `poolboy` and `riakc` had these errors.

As I see it, there are three fixes for these compiler errors:

1. Changing `queue()` for `queue:queue()` in lines 16 and 17 of poolboy.erl. This would break `poolboy` for older Erlang versions. I guess is the only solution when R18 arrives though.

2. Saying `poolboy` is not compatible with R17 in the README.md.

3. Add `nowarn_deprecated_type` compiler flag to the rebar configuration. This would make `poolboy` compatible with R17 and previous Erlang versions as well (not compatible for R18, though). *IMHO I believe this is the best solution for now. Therefore this is the solution I propose with this pull request.*

This fix does not break any of the tests as expected.