Pin mdl to 0.5.0 to fix packer basebuild

This attempts to fix the Ubuntu 16.04 basebuild where mdl has been
updated from 0.5.0 to 0.8.0 but it's mixlib-cli dependency no longer
supports ruby 2.3 which comes in the Ubuntu 16.04 LF base image.
See SEBA-877 for details.

Change-Id: Ib0a3ef51cac1aec4a6a4ccf7f9ab0118079f5767
Signed-off-by: Ciprian Barbu <ciprian.barbu@enea.com>
diff --git a/packer/provision/basebuild.sh b/packer/provision/basebuild.sh
index 23fb474..e212118 100644
--- a/packer/provision/basebuild.sh
+++ b/packer/provision/basebuild.sh
@@ -154,7 +154,7 @@
 
     # install ruby gems
     gem install \
-        mdl
+        mdl -v 0.5.0
         # end of gem install list
 
     # install npm modules