Slightly increase the lower bound of the retransmission timeout

This fixes an issue with IE8/9 when using the `jsonp-polling` transport.

If the round trip time is very low the computed retransmission timeout is too low for IE8/9 and the connection fails.

In this gist https://gist.github.com/lpinca/9760159 i put a simple test case to reproduce the issue.
When the connection fails the close event shows the following info:
```
LOG: SimpleEvent(
  type=close,
  code=2000,
  reason=All transports failed,
  wasClean=false,
  last_event=SimpleEvent(type=close, code=2007, reason=Transport timeouted, wasClean=false)
)
```

Increasing the `rto` lower bound by few milliseconds fixes the issue.