[SPARK-20759] SCALA_VERSION in _config.yml should be consistent with pom.xml

[https://issues.apache.org/jira/browse/SPARK-20759](https://issues.apache.org/jira/browse/SPARK-20759)
SCALA_VERSION in _config.yml is 2.11.7, but 2.11.8 in pom.xml. So I think SCALA_VERSION in _config.yml should be consistent with pom.xml.

Author: liuzhaokun <liu.zhaokun@zte.com.cn>

Closes #17992 from liu-zhaokun/new.

LICENSE

@@ -249,11 +249,11 @@ The text of each license is also included at licenses/LICENSE-[project].txt.
         (Interpreter classes (all .scala files in repl/src/main/scala
         except for Main.Scala, SparkHelper.scala and ExecutorClassLoader.scala),
         and for SerializableMapWrapper in JavaUtils.scala)
-     (BSD-like) Scala Actors library (org.scala-lang:scala-actors:2.11.7 - http://www.scala-lang.org/)
-     (BSD-like) Scala Compiler (org.scala-lang:scala-compiler:2.11.7 - http://www.scala-lang.org/)
-     (BSD-like) Scala Compiler (org.scala-lang:scala-reflect:2.11.7 - http://www.scala-lang.org/)
-     (BSD-like) Scala Library (org.scala-lang:scala-library:2.11.7 - http://www.scala-lang.org/)
-     (BSD-like) Scalap (org.scala-lang:scalap:2.11.7 - http://www.scala-lang.org/)
+     (BSD-like) Scala Actors library (org.scala-lang:scala-actors:2.11.8 - http://www.scala-lang.org/)
+     (BSD-like) Scala Compiler (org.scala-lang:scala-compiler:2.11.8 - http://www.scala-lang.org/)
+     (BSD-like) Scala Compiler (org.scala-lang:scala-reflect:2.11.8 - http://www.scala-lang.org/)
+     (BSD-like) Scala Library (org.scala-lang:scala-library:2.11.8 - http://www.scala-lang.org/)
+     (BSD-like) Scalap (org.scala-lang:scalap:2.11.8 - http://www.scala-lang.org/)
      (BSD-style) scalacheck (org.scalacheck:scalacheck_2.11:1.10.0 - http://www.scalacheck.org)
      (BSD-style) spire (org.spire-math:spire_2.11:0.7.1 - http://spire-math.org)
      (BSD-style) spire-macros (org.spire-math:spire-macros_2.11:0.7.1 - http://spire-math.org)

docs/_config.yml

@@ -17,7 +17,7 @@ include:
 SPARK_VERSION: 2.3.0-SNAPSHOT
 SPARK_VERSION_SHORT: 2.3.0
 SCALA_BINARY_VERSION: "2.11"
-SCALA_VERSION: "2.11.7"
+SCALA_VERSION: "2.11.8"
 MESOS_VERSION: 1.0.0
 SPARK_ISSUE_TRACKER_URL: https://issues.apache.org/jira/browse/SPARK
 SPARK_GITHUB_URL: https://github.com/apache/spark

external/docker/spark-test/base/Dockerfile

@@ -25,7 +25,7 @@ RUN apt-get update && \
     apt-get install -y less openjdk-7-jre-headless net-tools vim-tiny sudo openssh-server && \
     rm -rf /var/lib/apt/lists/*
 
-ENV SCALA_VERSION 2.11.7
+ENV SCALA_VERSION 2.11.8
 ENV CDH_VERSION cdh4
 ENV SCALA_HOME /opt/scala-$SCALA_VERSION
 ENV SPARK_HOME /opt/spark