Finalize the switch to Jenkinsfiles

To be done after and

• Change the CI URL everywhere (in particular in the POM): wherever we link to http://ci.hibernate.org/job/hibernate-search-master/, we should link to http://ci.hibernate.org/job/hibernate-search/ instead

• Enable Travis for the main repository, but not for PRs. Just to check that the .travis.yml file is still correct.

• Simplify .travis.yml:

  • Only one job per build (do not test multiple Elasticsearch versions)

  • Remove DB-specific configuration

  • Remove sonar-related configuration

  • Remove coveralls-related configuration

• Disable older Jenkins jobs for the "master" branch and rename them to be clear these are legacy jobs, or maybe even delete them if we're confident enough. Do not touch the release job, or any job related to maintenance branches.

• Edit the hibernate-search-pr job to only include PRs to maintenance branches which don't have a Jenkinsfile. PRs to master and newer branches don't need this job.