Support for tag-driven publishing on Travis.

Modify travis build to derive the project version from TRAVIS_TAG (when set).
as well as to have sbt run the `publish-signed` task if the environment is right.
The tag must match the semver regex `^v[0-9]+\.[0-9]+\.[0-9]+(-[A-Za-z0-9-]+)?`,
and the jdk we're running on must match PUBLISH_JDK (set in .travis.yml).

For every repo, you must create the following files (not included here)
according to the instructions in admin/gpg.sbt:
 - admin/pubring.asc      (commit)
 - admin/secring.asc      (DO NOT COMMIT)
 - sensitive.sbt          (DO NOT COMMIT)
    - passphrase for secring.asc
	- api token for staging to sonatype

Generate these using `admin/encryptAll.sh`:
 - admin/secring.asc.enc  (commit)
 - sensitive.sbt.enc      (commit)

Note that `encryptAll.sh` spits out a "- secure .... " line,
that you need to add to `.travis.yml`'s `env` section, so that
travis can supply the SECRET env variable, which is used to
decrypt the sensitive files above.
Secure config for publishing to sonatype
Merge pull request #39 from adriaanm/autopublish

Support for tag-driven publishing on Travis.