Removing JUnit dependency.

It looks like this is a left over or a mistake.
We haven't been shipping junit.jar in the hudson.war, and
core has no compile time dependency to junit classes.

core/pom.xml

@@ -648,6 +648,7 @@ THE SOFTWARE.
       <groupId>junit</groupId>
       <artifactId>junit</artifactId>
       <version>4.3.1</version>
+      <scope>test</scope>
     </dependency>
     <dependency><!-- needed by Jelly -->
       <groupId>javax.servlet</groupId>