Use built-in JSON module if available

This is a bajillion-x speed improvement in some situations.

Specifically, this can dramatically reduce parsing time for responses with lots of logs from pgbackups. In this particular case, it took parsing from 10+ minutes:

```console
2013-08-28 13:13:28 -0700: got body; decoding
2013-08-28 13:23:53 -0700: decoding completed
```
...to sub-second:
```console
2013-08-28 13:47:08 -0700: got body; decoding
2013-08-28 13:47:08 -0700: decoding completed
```

Log times are from instrumented calls to `get_backup` in `lib/heroku/client/pgbackups.rb`.