Add option to disable warning of extraneous ruby- prefix in version

Set `RBENV_VERSION_PREFIX_NOWARN`, for example in your `~/.zshrc` or `~/.bashrc`, to disable warnings of the extraneous `ruby-` prefix in `.ruby-version` files.

rbenv's behaviour is inconsitent with other Ruby version managers. If you work on projects with others who use a Ruby version manager that prefers the `ruby-` prefix, you will see this warning every time you run a shell command in a Ruby project that has the prefix.

Further discussion of the warning exists in https://github.com/sstephenson/rbenv/issues/543, such as whether the warning should exist or not in the first place. This just adds the option to disable it, leaving the default functionality as is.