support re-publishing an already tagged version for a new Scala version

[SethTisue]

suppose you've already published version 0.3.0 of your module, but now Scala 2.12.0-M5 just came out and you want to publish again for M5 only.

@adriaanm suggests it could work as follows: you push a tag of the form v0.3.0_2.12.0-M5. we'll need to add new logic in admin/build.sh (which unfortunately each module repo has its own copy of) to recognize this as a special form of version tag and do the right thing.

we can wait and face this the first time it actually comes up.

[SethTisue]

@lrytz done, right?