Add Ruby version to twilio-ruby user agent

Example headers:

```
Authorization: Basic QUN4eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4eHh4eDp5eXl5eXl5eXl5eXl5eXl5eXl5eXl5eXl5eXl5eXl5
Content-Length: 55
Accept-Charset: utf-8
User-Agent: twilio-ruby/3.11.4 (Ruby 2.0.0.247)
Connection: close
Host: api.twilio.com
Accept: application/json
Content-Type: application/x-www-form-urlencoded
Accept-Encoding: gzip;q=1.0,deflate;q=0.6,identity;q=0.3
```