Update ruby-build to 20190320

This should allow installation of ruby 2.5.5 and truffleruby-1.0.0-rc14

https://github.com/rbenv/ruby-build/releases/tag/v20190320

lib/utils.sh

-RUBY_BUILD_VERSION="20190314"
+RUBY_BUILD_VERSION="20190320"
 RUBY_BUILD_TAG="v$RUBY_BUILD_VERSION"
 
 echoerr() {