Compile and run Elasticsearch with JDK 12

With this commit we set the required JDK for compiling Elasticsearch to
JDK 12 and also allow to run it with JDK 12.

Relates #22

cars/v1/vanilla/config.ini

@@ -5,6 +5,6 @@ artifact_path_pattern = distribution/archives/oss-linux-tar/build/distributions/
 release_url = https://artifacts.elastic.co/downloads/elasticsearch/elasticsearch-oss-{{VERSION}}-linux-x86_64.tar.gz
 docker_image=docker.elastic.co/elasticsearch/elasticsearch-oss
 # major version of the JDK that is used to build Elasticsearch
-build.jdk = 11
+build.jdk = 12
 # list of JDK major versions that are used to run Elasticsearch
-runtime.jdk = 11,10,9,8
+runtime.jdk = 12,11,10,9,8