Fixes an issue with wrong type-parameter passed to getOption

The current implementation always fails with:
```
java.lang.ClassCastException: java.lang.String cannot be cast to scala.runtime.Nothing$
	at com.twitter.finagle.postgres.Row.get(Client.scala:152)
```

Somebody has forgotten to pass the type-parameter to the `getOption` function. The compiler used `Nothing` as default type.