Replace 'false' event handlers with 'undefined'

[apollocoder]

It seems the browser doesn't appreciate "false" as an event handler. When i set the disabled prop to true I get the following error in the console:

Warning: Expected `onScroll` listener to be a function, instead got `false`.
If you used to conditionally omit it with onScroll={condition && value}, pass onScroll={condition ? value : undefined} instead.
    in div (created by styled.div)
    in styled.div (created by PullRefresh)
    in PullRefresh (at StandbyView.js:25)
    in StandbyView (created by DragDropContext(StandbyView))
    in DragDropContext(StandbyView) (created by Connect(DragDropContext(StandbyView)))
    in Connect(DragDropContext(StandbyView)) (at App.js:31)
    in div (at App.js:30)
    in div (at App.js:27)
    in div (at App.js:25)
    in MuiThemeProvider (created by MuiThemeProviderWrapper)
    in MuiThemeProviderWrapper (at App.js:24)
    in App (created by Connect(App))
    in Connect(App) (at index.js:22)
    in Provider (at index.js:21)

Fortunately the message already delivers the solution. I implemented the small changes in this pull request.

Thank you very much for your work!

Best Regards

[eugenkiss]

@YusukeShibata this also fixes #28.

[yusukeshib]

Thanks @apollocoder , checked and merged.