Replace deprecated get_or_create_global_step

[martinkersner]

https://www.tensorflow.org/api_docs/python/tf/contrib/framework/get_or_create_global_step

tf.contrib.framework.get_or_create_global_step -> tf.train.get_or_create_global_step

[tensorflow-jenkins]

Can one of the admins verify this patch?

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[lukaszkaiser]

Looks good (for my part).

[martinkersner]

I signed it!

[googlebot]

CLAs look good, thanks!

[nealwu]

Looks good, thanks!