I have followed the instructions in the README, however, I'm still getting the error "You need to set $TM_RUBY variable." when I attempt to preview a file in TextMate. Here's the output from the terminal to verify that the TM_RUBY variable has been set:

donturner at donturner-macbookpro2 in ~/Library/Application Support/TextMate/Bundles/GFM-Preview.tmbundle/Support on master [!?]
$ defaults write com.macromates.textmate environmentVariables -array-add '{enabled = 1; value = "/Users/donturner/homebrew/bin/ruby"; name = "TM_RUBY"; }'

donturner at donturner-macbookpro2 in ~/Library/Application Support/TextMate/Bundles/GFM- Preview.tmbundle/Support on master [!?]
$ defaults read com.macromates.textmate environmentVariables
( 
    {
        enabled = 1;
        name = "TM_RUBY";
        value = "/Users/donturner/homebrew/bin/ruby";
    }
)

Any ideas what could be causing this?