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
if the user changes their version and then "control start" gets run, a new vertx version will get downloaded, but the conf directory from that new version won't be used.

which i guess is good since the user might have customized the contents, but might be bad if the new version expects something in particular from that directory.

make sense?