add ruby and install markdownlint via gem

Change-Id: I22fe39f59843d376ca390477d1127c683fb1cd04
diff --git a/packer/provision/basebuild.sh b/packer/provision/basebuild.sh
index d86c621..78d2fa1 100644
--- a/packer/provision/basebuild.sh
+++ b/packer/provision/basebuild.sh
@@ -67,6 +67,7 @@
         python-openssl \
         python-pip \
         python-urllib3 \
+        ruby \
         ssh \
         sshpass \
         zip
@@ -93,6 +94,11 @@
         virtualenv
         # end of pip install list
 
+    # install ruby gems
+    gem install \
+        mdl
+        # end of gem install list
+
     # install npm modules
     npm install -g \
         gitbook-cli \