Remove duplicate tokio and postgres dependencies

```
$ cargo update -p postgres:0.17.5
    Updating crates.io index
    Removing crypto-mac v0.8.0
    Removing hmac v0.8.1
    Removing mio-uds v0.6.8
    Removing pin-project-lite v0.1.5
    Removing postgres v0.17.5
    Removing postgres-protocol v0.5.2
    Removing postgres-types v0.1.3
    Removing tokio v0.2.22
    Removing tokio-postgres v0.5.5
    Removing tokio-util v0.3.1
```

Author: jyn514