Update readme.md for RVM

README.md

@@ -158,7 +158,7 @@ These derived images include a set of standard capabilities that enable many of
 - Node.js 8.11.2 LTS
 - Powershell Core v6.1.0-preview.2
 - Python 2.7.14, 3.3.7, 3.4.8, 3.5.5, 3.6.4 and 3.7.0-b2 (from hosted tools cache)
-- Ruby 2.3.7, 2.4.4 and 2.5.1 (from hosted tools cache)
+- Ruby 2.3.7, 2.4.4 and 2.5.1 (from the [tool cache](https://github.com/Microsoft/vsts-tasks/blob/master/docs/tools.md#tool-cache)) and RVM 1.29
 - Scala sbt-extras
 - Subversion 1.9.3