SQLite3 3.6.8+ supports nested transactions [ci skip]

http://www.sqlite.org/lang_savepoint.html
https://github.com/rails/rails/blob/master/activerecord/lib/active_record/connection_adapters/sqlite3_adapter.rb#L130-L132
Merge pull request #9677 from neerajdotname/update-comments-about-kotori

SQLite3 3.6.8+ supports nested transactions [ci skip]