remove "-XX:+UseCompressedStrings" option

remove "-XX:+UseCompressedStrings" option from tuning guide since jdk7 no longer supports this.
Merge pull request #439 from CrazyJvm/master

SPARK-1024 Remove "-XX:+UseCompressedStrings" option from tuning guide

remove "-XX:+UseCompressedStrings" option from tuning guide since jdk7 no longer supports this.