The version number of Spark in docs/_config.yaml for master branch should be 1.2.0 for now.

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

Closes #2943 from sarutak/SPARK-4089 and squashes the following commits:

aba7fb4 [Kousuke Saruta] Fixed the version number of Spark in _config.yaml