Switch to 7.9 LST as minimal supported version

[romani]

New LST release 7.9
https://www.sonarqube.org/downloads/

We need to update our CI https://github.com/checkstyle/sonar-checkstyle/blob/master/wercker.yml#L1

And this will be our minimal supported version.

Examples of orchestrate usage :
https://github.com/SonarSource/sonar-java/blob/5aaaeba90cd85a40391706ad2f806d6a156f80cb/its/ruling/src/test/java/org/sonar/java/it/JavaRulingTest.java
https://github.com/SonarSource/sonar-jacoco/blob/master/its/src/test/java/org/sonar/plugins/jacoco/its/JacocoTest.java

[romani]

There was error:

[INFO] /home/rivanov/java/github/checkstyle/sonar-checkstyle/src/test/java/
org/sonar/plugins/checkstyle/CheckstyleExecutorTest.java: Recompile 
with -Xlint:deprecation for details.
[INFO] -------------------------------------------------------------
[ERROR] COMPILATION ERROR : 
[INFO] -------------------------------------------------------------
[ERROR] /home/rivanov/java/github/checkstyle/sonar-checkstyle/src/test/
java/org/sonar/plugins/checkstyle/CheckstyleProfileExporterTest.java:[297,20] 
org.sonar.plugins.checkstyle.CheckstyleProfileExporterTest.TestActiveRule 
is not abstract and does not override abstract method qpKey() in 
org.sonar.api.batch.rule.ActiveRule
[INFO] 1 error

[romani]

Docker image for 7.9 was tested on latest plugin version:

✔ ~/java/github/checkstyle/sonar-checkstyle [master|✚ 4] 
13:54 $ git diff src/main/java/org/sonar/plugins/checkstyle/CheckstyleAuditListener.java
diff --git a/src/main/java/org/sonar/plugins/checkstyle/CheckstyleAuditListener.java b/src/main/java/org/sonar/plugins/checkstyle/CheckstyleAuditListener.java
index c3c7d85..92260f4 100644
--- a/src/main/java/org/sonar/plugins/checkstyle/CheckstyleAuditListener.java
+++ b/src/main/java/org/sonar/plugins/checkstyle/CheckstyleAuditListener.java
@@ -49,6 +49,7 @@ public class CheckstyleAuditListener implements AuditListener {
     private static final Logger LOG = LoggerFactory.getLogger(CheckstyleAuditListener.class);
 
     private final ActiveRules ruleFinder;
+    private ActiveRules RRRuleFinder;
     private final FileSystem fileSystem;
 
     private InputFile currentResource;

I need to push changes for Docker first in master and SHA commit is used in name for docker image in dockerhub, based on https://github.com/checkstyle/sonar-checkstyle/wiki/How-to-push-Dockerfile-to-docker-hub.

new docker image was pushed - https://hub.docker.com/layers/checkstyle/sonarqube-maven-git/7.9-community-5302c42/images/sha256-4e26dc5869f273b8579fcf23ebfcb786e677cb050972cf16fe3db134b3ddee59