checkpoint_dir not found when test_max_length is greater than max_length

For example, assume max_length is 10 and test_max_length is 20:

Percent: [####################] 100.00% Finished.
 [*] Build a NTM model finished
 [*] Reading checkpoints...
Traceback (most recent call last):
  File "main.py", line 49, in <module>
    tf.app.run()
  File "/usr/local/lib/python2.7/dist-packages/tensorflow/python/platform/app.py", line 30, in run
    sys.exit(main(sys.argv))
  File "main.py", line 38, in main
    ntm.load(FLAGS.checkpoint_dir, 'copy')
  File "/root/NTM-tensorflow/ntm.py", line 257, in load
    raise Exception(" [!] Testing, but %s not found" % checkpoint_dir)
Exception:  [!] Testing, but checkpoint/copy_20 not found

Make a symbolic link from checkpoint/copy_10 to checkpoint/copy_20 works around the problem.

Nice job, thank you!