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

pdalpra/sbt,sbt/sbt,xuwei-k/xsbt,Sciss/sbt,twitter-forks/sbt,mdedetrich/sbt,xeno-by/old-scalameta-sbt,dyx/sbt,jasonchaffee/sbt,mdedetrich/sbt,xeno-by/old-scalameta-sbt,adriaanm/sbt,jaceklaskowski/sbt,jaceklaskowski/sbt,Duhemm/sbt,Duhemm/sbt,xuwei-k/xsbt,sbt/sbt,sbt/sbt,sbt/sbt,sbt/sbt,dyx/sbt,mdedetrich/sbt,jaceklaskowski/sbt,pocketberserker/sbt,sbt/sbt,dyx/sbt,som-snytt/xsbt,dyx/sbt,mdedetrich/sbt,sbt/sbt,dansanduleac/sbt,Duhemm/compiler-bridge,dyx/sbt,twitter-forks/sbt,mdedetrich/sbt,twitter-forks/sbt,jaceklaskowski/sbt,xeno-by/old-scalameta-sbt,twitter-forks/sbt,mdedetrich/sbt,jkinkead/sbt,mdedetrich/sbt,xuwei-k/xsbt,jkinkead/sbt,xeno-by/old-scalameta-sbt,corespring/sbt,jkinkead/sbt,twitter-forks/sbt,xeno-by/old-scalameta-sbt,xeno-by/old-scalameta-sbt,jkinkead/sbt,jkinkead/sbt,xuwei-k/xsbt,jaceklaskowski/sbt,xuwei-k/xsbt