Incorrect variant for answer distribution bug fix

This request fixes an issue we have noticed in which an answer's variant is not correctly reflected in the pipeline.  The cause of the bug is a conditional which assumes that an answer's variant cannot be changed over the duration of a course and therefore relies on the variant value for the most recent answer to fill in those of the rest.  This bug has been causing issues with inline analytics display (referenced <a href="https://github.com/Stanford-Online/edx-platform/pull/165">here</a>) specifically when questions which had had randomization enabled in the past and then disabled displayed incorrectly, as the older answers to the randomized version were reported to be non-randomized by the pipeline.

The changes include a small removal in the answer_dist task and alteration of a related test case.  There was a comment in the test case regarding the behavior which I have altered, so please let me know if the prior behavior was actually desired anywhere.  As always, feedback is encouraged.  @jbau @dcadams @mulby @sarina