modified SparkPluginBuild.scala to use https protocol for accessing gith...

We cannot build Spark behind a proxy although we execute sbt with -Dhttp(s).proxyHost -Dhttp(s).proxyPort -Dhttp(s).proxyUser -Dhttp(s).proxyPassword options.
It's because of using git protocol to clone junit_xml_listener.git.
I could build after modifying SparkPluginBuild.scala.

I reported this issue to JIRA.
https://spark-project.atlassian.net/browse/SPARK-1046