No special treatment for value

`POST element/:id/value` is a thing, the same isn't true for `GET`.

**Reference**

* [POST /value](https://code.google.com/p/selenium/wiki/JsonWireProtocol#/session/:sessionId/element/:id/value)
* [GET /text](https://code.google.com/p/selenium/wiki/JsonWireProtocol#/session/:sessionId/element/:id/text)
* [GET /attribute/:name](https://code.google.com/p/selenium/wiki/JsonWireProtocol#GET_/session/:sessionId/element/:id/attribute/:name)
Remove fragile trailing slash
Merge pull request #17 from groupon-testium/jk/value-is-an-attribute

No special treatment for value