Added release notes for 2.1.1.
Added release note for 2.1.2.
Added release notes for 2.1.3, and make it to display at the top in landing page.
Added 2.1.1, 2.1.2 and 2.1.3 release to Download page.
Added 2.1.1, 2.1.2 and 2.1.3 to scaladoc page.
Merge branch '2.1.x-update' of https://github.com/cheeseng/scalatest-website
Tweaked release notes for 2.1.1, 2.1.2, and 2.1.3.