Remark that os gem is needed

make.sh

@@ -75,7 +75,7 @@ rule() {
     ;; setup)
       echo "Make sure you have the following installed: opam >= 2.0.0, git, patch, m4, autoconf, libgmp-dev, libmpfr-dev, pkg-config"
       echo "For the --html output you also need: javac, ant, dot (graphviz)"
-      echo "For running the regression tests you also need: ruby, gem, curl"
+      echo "For running the regression tests you also need: ruby, gem, curl, and the `os` gem"
       echo "For reference see ./Dockerfile or ./scripts/travis-ci.sh"
       opam_setup
     ;; dev)