Fix calculation of estimated time remaining

[epwalsh]

This line:

allennlp/allennlp/training/trainer.py

Line 763 in 6b37dd2

formatted_time = time.strftime("%H:%M:%S", time.gmtime(estimated_time_remaining))

calculates (or incorrectly formats) the wrong estimated time remaining when estimated_time_remaining is greater than 1 day, since time.gmtime interprets the number of seconds as time since the epoch, not a time duration.

[matt-gardner]

Thanks!