Bump version to 0.13 (#2460)

Summary: Pull Request resolved: https://github.com/pytorch/audio/pull/2460

Reviewed By: nateanl, mthrok

Differential Revision: D36992043

Pulled By: hwangjeff

fbshipit-source-id: 3a2a7f8991beaeaa2af0f620985230a68df201c2

version.txt

-0.12.0a0
+0.13.0a0