[SPARK-8574] org/apache/spark/unsafe doesn't honor the java source/ta…

…rget versions.

I basically copied the compatibility rules from the top level pom.xml into here.  Someone more familiar with all the options in the top level pom may want to make sure nothing else should be copied on down.

With this is allows me to build with jdk8 and run with lower versions.  Source shows compiled for jdk6 as its supposed to.

Author: Tom Graves <tgraves@yahoo-inc.com>
Author: Thomas Graves <tgraves@staydecay.corp.gq1.yahoo.com>

Closes #6989 from tgravescs/SPARK-8574 and squashes the following commits:

e1ea2d4 [Thomas Graves] Change to use combine.children="append"
150d645 [Tom Graves] [SPARK-8574] org/apache/spark/unsafe doesn't honor the java source/target versions

unsafe/pom.xml

@@ -80,7 +80,7 @@
           <groupId>net.alchim31.maven</groupId>
           <artifactId>scala-maven-plugin</artifactId>
           <configuration>
-            <javacArgs>
+            <javacArgs combine.children="append">
               <!-- This option is needed to suppress warnings from sun.misc.Unsafe usage -->
               <javacArg>-XDignore.symbol.file</javacArg>
             </javacArgs>