Roll back sbt/sbt@482f99 (aggressive deletion of cache). Fixes #1091.

* sbt 0.12.3 introduced sbt/sbt@482f99 to fix #637 and #641.
* The underlying issue of #641 was an incorrect classifier, and it was
  fixed in sbt/sbt@718fa9 for #683 and shipped as sbt 0.13.0.
Merge pull request #1234 from sbt/wip/undo-nonsnapshot-deletion

Roll back aggressive deletion of cache. Fixes #1091.