Support tag driven publishing

I started with:

   https://github.com/scala/scala-swing/commit/a6a8e1d77e#diff-3acefdae08499733e855dd23e1af933dR8

And:
  - modified the regex that derives the version from the tag to
    handle the fact we have the scala binary version appended to
    our tags
  - Removed unnessary cross building with Scala 2.11 (as this is
    on the 2.10.x branch)
  - generated a key pair for the async repository
  - added passphrase for that key, and sonatype credentials, to
    sensitive.sbt
  - encrypted the secrets with the travis key.
Merge pull request #100 from retronym/topic/tag-driven-release

Support tag driven publishing