UseStringDeduplication is only available in Java 8

Signed-off-by: Holger Levsen <holger@layer-acht.org>

hosts/jenkins/etc/default/jenkins

@@ -10,7 +10,7 @@ JAVA=/usr/bin/java
 #JAVA_ARGS="-Djava.awt.headless=true"  # Allow graphs etc. to work even when an X server is present
 #JAVA_ARGS="-Xmx256m"
 #JAVA_ARGS="-Djava.net.preferIPv4Stack=true" # make jenkins listen on IPv4 address
-JAVA_ARGS="-Djava.awt.headless=true -Xms4G -Xmx8G -server -XX:+AlwaysPreTouch -XX:MaxPermSize=384m -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/var/lib/jenkins/heapDumps/ -Xloggc:$JENKINS_HOME/gc-%t.log -XX:NumberOfGCLogFiles=5 -XX:+UseGCLogFileRotation -XX:GCLogFileSize=20m -XX:+PrintGC -XX:+PrintGCDateStamps -XX:+PrintGCDetails -XX:+PrintHeapAtGC -XX:+PrintGCCause -XX:+PrintTenuringDistribution -XX:+PrintReferenceGC -XX:+PrintAdaptiveSizePolicy -XX:+UseG1GC -XX:+ExplicitGCInvokesConcurrent -XX:+ParallelRefProcEnabled -XX:+UseStringDeduplication -XX:+UnlockExperimentalVMOptions -XX:G1NewSizePercent=20 -XX:+UnlockDiagnosticVMOptions -XX:G1SummarizeRSetStatsPeriod=1"
+JAVA_ARGS="-Djava.awt.headless=true -Xms4G -Xmx8G -server -XX:+AlwaysPreTouch -XX:MaxPermSize=384m -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/var/lib/jenkins/heapDumps/ -Xloggc:$JENKINS_HOME/gc-%t.log -XX:NumberOfGCLogFiles=5 -XX:+UseGCLogFileRotation -XX:GCLogFileSize=20m -XX:+PrintGC -XX:+PrintGCDateStamps -XX:+PrintGCDetails -XX:+PrintHeapAtGC -XX:+PrintGCCause -XX:+PrintTenuringDistribution -XX:+PrintReferenceGC -XX:+PrintAdaptiveSizePolicy -XX:+UseG1GC -XX:+ExplicitGCInvokesConcurrent -XX:+ParallelRefProcEnabled -XX:+UnlockExperimentalVMOptions -XX:G1NewSizePercent=20 -XX:+UnlockDiagnosticVMOptions -XX:G1SummarizeRSetStatsPeriod=1"
 
 PIDFILE=/var/run/$NAME/$NAME.pid