Set missing selendroid basic capabilities

I found that SelendroidCapabilities(String aut) and SelendroidCapabilities(String serial, String aut) constructors don't set basic selendroid capabilities that set by default constructor (browserName, platformName and autonationName).

I'm not sure this is a bug or not.

If it's a bug, please pull this PR.

Not that if merge this PR, grid documentation (http://selendroid.io/scale.html) should also be update to
include "platformName" capability for selendroid capabilities like as follows:
<pre>
      "browserName": "selendroid",      
      "platformName": "android",      
      "maxInstances": 1,
      "aut": "io.selendroid.testapp:0.11.0"
  </pre>

Regards.