fix logging with only one eval metric
fix logging with only one eval metric

ufal/neuralmonkey,juliakreutzer/bandit-neuralmonkey,bastings/neuralmonkey,juliakreutzer/bandit-neuralmonkey,ufal/neuralmonkey,bastings/neuralmonkey,juliakreutzer/bandit-neuralmonkey,ufal/neuralmonkey,juliakreutzer/bandit-neuralmonkey,bastings/neuralmonkey,ufal/neuralmonkey,juliakreutzer/bandit-neuralmonkey,ufal/neuralmonkey,bastings/neuralmonkey,bastings/neuralmonkey