Work around for the slowly-updating test

The 2nd test "should follow a slowly-updating file and emit lines as events" was failing due do the test file being truncated.  

"File truncated, seeking to beginning: /var/folders/xm/4mnw8l_n10j3cc3trf484ym40000gn/T/lumberjack-test-file20130916-42600-15laykt"

By adding the @file.size it seems not to truncate the file.