Fix broken `brew cask alfred link`

`brew cask alfred link` is broken because `features.defaultresults.scope` is a string instead of a dictionary:

    [00:13]:) ~ $ brew cask alfred link
    ==> Successfully linked Alfred to homebrew-cask.
    [00:13]:) ~ $ brew cask alfred status
    ==> Alfred is not linked to homebrew-cask.
    [00:15]:) ~ $ defaults read com.runningwithcrayons.Alfred-Preferences
    {
        NSNavLastRootDirectory = "~/Documents";
        NSNavPanelExpandedSizeForOpenMode = "{750, 480}";
        "NSWindow Frame alfredpreferences" = "0 56 1383 822 0 0 1440 878 ";
        "features.defaultresults.scope" = "('/Applications','/Applications/Xcode.app/Contents/Applications','/Developer/Applications','/Library/PreferencePanes','/System/Library/PreferencePanes','~/Library/Caches/Metadata/','~/Library/Mobile Documents/','~/Library/PreferencePanes','/opt/homebrew-cask/Caskroom')";
        lastTab = features;
        lastTabBuild = 227;
        lastTabTS = "415265223.980409";
        version = "2.1.1";
    }

- Remove quotes from `defaults write` args. With quotes the value is
  written as a string instead of a dictionary, which breaks the search
  scope in alfred.
- Make the default scope consistent with alfred default