[DOC] Fixed Mesos version in doc from 0.18.1 to 0.21.0

#3934 upgraded Mesos version so we should also fix docs right?

This issue is really minor so I don't file in JIRA.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #3982 from sarutak/fix-mesos-version and squashes the following commits:

9a86ee3 [Kousuke Saruta] Fixed mesos version from 0.18.1 to 0.21.0

docs/_config.yml

@@ -17,6 +17,6 @@ SPARK_VERSION: 1.3.0-SNAPSHOT
 SPARK_VERSION_SHORT: 1.3.0
 SCALA_BINARY_VERSION: "2.10"
 SCALA_VERSION: "2.10.4"
-MESOS_VERSION: 0.18.1
+MESOS_VERSION: 0.21.0
 SPARK_ISSUE_TRACKER_URL: https://issues.apache.org/jira/browse/SPARK
 SPARK_GITHUB_URL: https://github.com/apache/spark