imageoptim: Only link ImageOptim.app

Before:

    $ brew cask install imageoptim
    ==> Downloading http://imageoptim.com/ImageOptim.tbz2
    ######################################################################## 100.0%
    ==> Success! imageoptim installed to /opt/homebrew-cask/Caskroom/imageoptim/1.4.0
    ==> Linking ImageOptim.app to /Users/Mathias/Applications/ImageOptim.app
    ==> Linking finish_installation.app to /Users/Mathias/Applications/finish_installation.app

After:

    $ brew cask install imageoptim
    ==> Downloading http://imageoptim.com/ImageOptim.tbz2
    ######################################################################## 100.0%
    ==> Success! imageoptim installed to /opt/homebrew-cask/Caskroom/imageoptim/1.4.0
    ==> Linking ImageOptim.app to /Users/Mathias/Applications/ImageOptim.app