De acordo com os documentos oficiais , parece que você precisa prefixar seus scripts como este :
#!/usr/bin/env ruby
E, em seguida, faça uma das seguintes ações para informar rbenv qual versão do Ruby usar:
trecho: link
Choosing the Ruby Version
When you execute a shim,
rbenvdetermines which Ruby version to use by reading it from the following sources, in this order:
The
RBENV_VERSIONenvironment variable, if specified. You can use therbenvshell command to set this environment variable in your current shell session.The first
.ruby-versionfile found by searching the directory of the script you are executing and each of its parent directories until reaching the root of your filesystem.The first
.ruby-versionfile 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-versionfile in the current working directory with therbenvlocal command.The global
~/.rbenv/versionfile. You can modify this file using therbenvglobal command. If the global version file is not present,rbenvassumes you want to use the "system" Ruby—i.e. whatever version would be run ifrbenvweren't in your path.
Você pode usar este comando para criar um arquivo .ruby-version no diretório junto com o script Ruby.
$ rbenv local 1.9.3-p327