minor updates to build and release

Hopefully this will get the build working right:

* Fixes: Invalid POM: /com/netflix/atlas/atlas-json/1.4.1/atlas-json-1.4.1.pom: Developer information missing
* Fixes: Missing Signature: '/com/netflix/atlas/atlas-json/1.4.1/atlas-json-1.4.1.pom.asc' does not exist for 'atlas-json-1.4.1.pom'.
* Update sbt-pgp version
Merge pull request #35 from brharrington/sbt-pgp-version

minor updates to build and release