Added imagemin-gifsicle npm module to package.json.

...n grunt build.

$ grunt build
    Loading "imagemin.js" tasks...ERROR
    >> Error: Cannot find module 'imagemin-gifsicle'


    Running "concurrent:dist" (concurrent) task
        Warning: Loading "imagemin.js" tasks...ERROR
        >> Error: Cannot find module 'imagemin-gifsicle'
        Warning: Task "imagemin" not found. Use --force to continue.
        
        Aborted due to warnings.
        
        
        Execution Time (2014-11-12 12:50:24 UTC)
        loading tasks  4ms  ▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇▇ 100%
        Total 4ms Use --force to continue.
            
            Aborted due to warnings.