Updated DataContext.js with up-to-date URL

The previous URL (`'https://' + this.appName + '.infusionsoft.com/api/xmlrpc'`) was returning an error when trying to use it: 

```
{ [Error: getaddrinfo ENOTFOUND] code: 'ENOTFOUND', errno: 'ENOTFOUND', syscall: 'getaddrinfo' }
```

updated the URL to the current URL as per the [documentation](https://developer.infusionsoft.com/docs/read/Getting_Started_With_OAuth2)

Additionally, further functions may be needed to use the user's unique access_token in place of the apiKey for making calls. Not sure how this will fit into all that, but this patch at least fixes this initial issue.