Removed the use of merge method as the default capabilities override cus...

The use of merge method causes  default browser capabilities override custom capabilities. For example, if my target browser is `ie` and I want to run tests on `Windows 7`, typical nemo configuration would be,
```
"nemoData": {
     "targetBrowser":"ie",
     "serverCaps" : {
         "username": "sauce_username",
         "password": "sauce_password",
         "platform" : "Windows 7"
      }
}
```
However the default 'ie' capabilities has platform set to 'WINDOWS' and the use of `merge` method below overrides custom capability platform `Windows 7` to `WINDOWS` which is not desired.
`caps.merge(webdriver.Capabilities[tgtBrowser]());`

So instead the change in this PR takes the default browser capabilities and modifies the object based on custom capabilities.