ScalaTest 2.1.5 is a maintenance release for the 2.1.x series that includes one bug fix: the addition of Java annotations for browser drivers that were accidentally left out of the 2.1.4 release. No source code using ScalaTest 2.0+ should break, but you will likely need to do a clean build to upgrade from 2.0. However, 2.1.5 is binary compatible with 2.1.0, 2.1.1, and 2.1.2, and 2.1.3, and 2.1.4, so you can just change the version number in your build to upgrade from earlier 2.1.x releases.
InternetExplorerBrowser, and >>>>
SafariBrowserthat were accidentally excluded from the 2.1.4 Jar file.