outdated use of GET element/:id/value

According to the [JsonWireProtocol](http://code.google.com/p/selenium/wiki/JsonWireProtocol#/session/:sessionId/element/:id/value) docs there is no GET method for `element/:id/value`. It was probably removed. Anyway, the replacement for it is [this](https://code.google.com/p/selenium/wiki/JsonWireProtocol#/session/:sessionId/element/:id/attribute/:name):

```
GET /session/:sessionId/element/:id/attribute/:name
```

We should update this in our protocol bindings as well before it is no more supported by selenium.