Publish Gatling to Sonatype OSS, close #1372

This PR makes the necessary changes to SBT build definition to publish to Sonatype repositories :

* Revert to Saxon published in Central, to comply to Sonatype OSS dependency policy
* Removal of the Excilys Nexus resolvers, since all required dependencies are hosted in Central
* Removal of the aether-deploy plugin, which was needed for publishing to Cloudbees snapshots repositories.
* Add the sbt-pgp plugin, to sign published artifacts (public GPG keys are needed)

This also fixes an issue we had with the aether-deploy plugin: JARs were published for `gatling-parent` and `gatling-parent`, even if JAR publishing was explicitly disabled for those two modules.