Update FailureDetector class of out_forward.rb

The method 'phi(now) of 'FailureDetector class calculates the variable 't wrong way.

When @heartbeat_interval is not the default value (1 sec), 't value contains @heartbeat_interval time. But 'mean doesn't. So @heartbeat_interval is set higher, 'phi(now) returns a higher value.
Merge pull request #140 from tsuda7/master

Calculation phi in out_forward.rb seems wrong.