Fix executable directory reference in rbenv rehash and rbenv which.

The ruby executable path has changed *recently* and this patch fixes the shims by using an updated path:

     ➜  ~ (home) rbenv shell rbx-2.0.0-rc1
     ➜  ~ (home) ruby -v
    rubinius 2.0.0rc1 (1.9.3 d19e086d 2012-11-02 JI) [x86_64-apple-darwin12.3.0]
     ➜  ~ (home) gem env
    RubyGems Environment:
      - RUBYGEMS VERSION: 1.8.24
      - RUBY VERSION: 1.9.3 (2012-11-02 patchlevel 0) [x86_64-apple-darwin12.3.0]
      - INSTALLATION DIRECTORY: /usr/local/var/rbenv/versions/rbx-2.0.0-rc1/gems/1.9
      - RUBYGEMS PREFIX: /usr/local/var/rbenv/versions/rbx-2.0.0-rc1
      - RUBY EXECUTABLE: /usr/local/var/rbenv/versions/rbx-2.0.0-rc1/bin/rbx
      - EXECUTABLE DIRECTORY: /usr/local/var/rbenv/versions/rbx-2.0.0-rc1/gems/1.9/bin
      - RUBYGEMS PLATFORMS:
        - ruby
        - x86_64-darwin-12
        - x86_64-rubinius-2.0
      - GEM PATHS:
         - /usr/local/var/rbenv/versions/rbx-2.0.0-rc1/gems/1.9
         - /Users/attila/.gem/rbx/1.9
      - GEM CONFIGURATION:
         - :update_sources => true
         - :verbose => true
         - :benchmark => false
         - :backtrace => false
         - :bulk_threshold => 1000
         - "gem" => "--no-ri --no-rdoc"
      - REMOTE SOURCES:
         - http://rubygems.org/