Adjust handling of --target-buildid for usage with Mozmill CI (#186)

This fixes issue #186. Please keep in mind that I created this PR for hotfix-2.0 given that Mozmill 2.1-dev doesn't work right now. I will make sure to land it first on default, and cherry-pick for hotfix-2.0. @andreeamatei or @andreieftimie can one of you have a look at it please?