Updates `last` and `export` commands to read from correct key.

Fixes #1155.

It seems that somehow during the 0.13.{1 -> 2 } transition, we
stopped pointing at the correct key for TaskKeys (either that or
task streams are now all associated with the `streams` key).  I
think this may have been inadvertently caused from several
refactorings to enable greater control over the execution of tasks.

This points the `last*` methods at the correct key for tasks,
fixing both `last <key>` and `export <key>` commands.
Merge pull request #1158 from sbt/wip/issue-1155

Updates `last` and `export` commands to read from correct key.