Fix compilation on Hadoop 2.4.x.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #483 from vanzin/yarn-2.4 and squashes the following commits:

0fc57d8 [Marcelo Vanzin] Fix compilation on Hadoop 2.4.x.

yarn/common/src/main/scala/org/apache/spark/deploy/yarn/ClientBase.scala

@@ -458,7 +458,8 @@ object ClientBase {
     }
 
     /** Add entry to the classpath. */
-    def addClasspathEntry(path: String) = Apps.addToEnvironment(env, Environment.CLASSPATH.name, path)
+    def addClasspathEntry(path: String) = YarnSparkHadoopUtil.addToEnvironment(env,
+      Environment.CLASSPATH.name, path, File.pathSeparator)
     /** Add entry to the classpath. Interpreted as a path relative to the working directory. */
     def addPwdClasspathEntry(entry: String) = addClasspathEntry(Environment.PWD.$() + Path.SEPARATOR + entry)