Fixing "undefined operation random(10) times 0.1"

`sass --pre` on Windows machines throws an exception when compiling CSS shake like so.

    Syntax error: Undefined operation: "random(10) times 0.1".
        on line 111 of D:/Laburo/vertical-server/5.0/vendor/csshake/sass/_mixins.scss, in `@content'
        from line 65 of D:/Laburo/vertical-server/5.0/vendor/csshake/sass/_mixins.scss, in `keyframes'
        from line 95 of D:/Laburo/vertical-server/5.0/vendor/csshake/sass/_mixins.scss, in `shake'
        from line 80 of D:/Laburo/vertical-server/5.0/vendor/csshake/sass/_shake.scss
        from line 2 of D:/Laburo/vertical-server/5.0/vendor/csshake/sass/csshake.scss
        from line 37 of app/styles/app.scss

This change, simple enough and equivalent, does not fail.