According to the official docs it sounds like you just need to prefix your scripts like this:
And then do one of the following things to tell
rbenv which version of Ruby to use:
Choosing the Ruby Version
When you execute a shim,
rbenv determines which Ruby version to use by reading it from the following sources, in this order:
RBENV_VERSION environment variable, if specified. You can use the
rbenv shell command to set this environment variable in your current shell session.
.ruby-version file found by searching the directory of the script you are executing and each of its parent directories until reaching the root of your filesystem.
.ruby-version file found by searching the current working directory and each of its parent directories until reaching the root of your filesystem. You can modify the
.ruby-version file in the current working directory with the
rbenv local command.
~/.rbenv/version file. You can modify this file using the
rbenv global command. If the global version file is not present,
rbenv assumes you want to use the "system" Ruby—i.e. whatever version would be run if
rbenv weren't in your path.
You can then use this command to create a
.ruby-version file in the directory along with the Ruby script.
$ rbenv local 1.9.3-p327