fix `rbenv()` function in ksh and dash
fix `rbenv()` function in ksh and dash

ksh syntax becomes:

    function rbenv {
      typeset command

`typeset` only declares a local variable if there's an explicit
`function` declaration; otherwise the variable leaks.

Other shells use this syntax:

    rbenv() {
      local command

This is for dash compatibility, which supports neither `function` nor
`typeset`.

references #205, fixes #408

cehoffman/rbenv,mediadoneright/rbenv,mediadoneright/rbenv,mediadoneright/rbenv,cehoffman/rbenv,cehoffman/rbenv