state_is_tuple=True: Fix training on 0.12

[julien-c]

This parallels the changes made to char-rnn-tensorflow here: sherjilozair/char-rnn-tensorflow@991704e
which in turn parallels changes made to TF tutorial ptb_word_lm.py

[pender]

Isn't this already the default value of the argument? Is there a style reason why it is preferred to make the argument explicit?

[julien-c]

Not sure, but it worked in my install (was failing without the change)

[pender]

Odd! Okay, well it can't hurt -- will merge it in. Thanks for the PR!