Add backup import for numpy.random.choice

numpy.random.choice is new to numpy version 1.7. This commit
adds support to numpy versions prior to 1.7. Based on used
made here, basic random.choice could have been used though.
Revert "Add backup import for numpy.random.choice"

This reverts commit 7c1f0d8fd9ba9bcd75574425217758edf70f03fc.
Changes numpy.random.choice to random.choice

There is no performance justification for using
numpy.random.choice.
Merge pull request #1 from bouthilx/clean-skipgram

Add backup import for numpy.random.choice