fixing issue in http-duplex

- the test for node 0.8 did not work therefore core streams were used at
  all times
- readable-stream wasn't installed, so even if test would have worked it
  would have failed when trying to require it
- now using readable-stream no matter what the node version is, in order
  to be consistent across all versions
- making it a dependency as well (anything that gets required, even
  conditonally, should be)

Here is the error I got without this fix when running in `node 0.8`:

```
stderr:  |
          util.js:538
            ctor.prototype = Object.create(superCtor.prototype, {
                                                    ^
          TypeError: Cannot read property 'prototype' of undefined
              at Object.exports.inherits (util.js:538:43)
              at Object.<anonymous> (/home/thlorenz/dev/systems/projects/dockops/node_modules/dockerode/node_modules/docker-modem/lib/http_duplex.js:8:6)
              at Module._compile (module.js:449:26)
              at Object.Module._extensions..js (module.js:467:10)
              at Module.load (module.js:356:32)
              at Function.Module._load (module.js:312:12)

▽
              at Module.require (module.js:362:17)
              at require (module.js:378:17)
              at Object.<anonymous> (/home/thlorenz/dev/systems/projects/dockops/node_modules/dockerode/node_modules/docker-modem/lib/modem.js:7:18)
              at Module._compile (module.js:449:26)
```