Fix backends support.

Hello,

I was using backends in 0.5.0 (with SBT 0.12) but when I tried switching to SBT 0.13 and sbt-appengine 0.6.0, appengine-deploy-backends started failing.

appcfgBackendTask was ignoring it's action parameter, causing
appcfg.sh to print usage information. Looks like the problem
was introduced in 50552f4954dfe69ba6365fdb3537efdd0efb1ba4.