Fix #11 - Make minor/milestone version updates easier

- Remove Vert.x minor/milestone version from the cartridge metadata
- Update setup/install/control scripts
- Update README, improve Updating Vert.x steps

https://github.com/vert-x/openshift-cartridge/issues/11
https://bugzilla.redhat.com/show_bug.cgi?id=1158736
it's probably better than overwriting their customizations so i guess this is the best solution, just wanted to make sure it was thought out.