Set java source version so sonar can deactivate rules accordingly

Starting from v3.7, the Java plugin is able to react to the java version
used for sources. This feature allows the deactivation of rules that
target higher versions of Java than the one that is in use so that false
positives aren't generated from irrelevant rules.

Change-Id: I90a19bcf55e081876d05842b8b611df74a4a5484

tools/gen_sonar_project_properties.py

diff --git a/tools/gen_sonar_project_properties.py b/tools/gen_sonar_project_properties.py
index 2ef679f..832b7f1 100755
--- a/tools/gen_sonar_project_properties.py
+++ b/tools/gen_sonar_project_properties.py
@@ -69,6 +69,7 @@
 sonar.sources=%s
 sonar.tests=%s
 sonar.sourceEncoding=UTF-8
+sonar.java.source=1.7
 sonar.java.binaries=%s
 
 sonar.junit.reportsPath=%s