Add a pubring and a secring using encoded Travis variables.

[alexflav23]

To automatically publish the code to the Central Repository through the existing Outworkers integration, we need to:

• Add the publishSigned command as part of the after_success lifecycle.
• Add the encrypted_key.asc through Travis encrypted files, containing a signing only copy of the master Outworkers PGP key.
• Add the sbt-sonatype plugin to allow publishing to Maven Central automatically.
• Add the sonatypeReleaseAll command after publishing to the intermediary repository.