Switching to flexboxgrid2

[Kamahl19]

Hi,

since the original flexboxgrid seems abandoned and has pretty major bugs (like this one kristoferjoseph/flexboxgrid#144), I think it might be better to switch to the forked and actively developed https://github.com/evgenyrodionov/flexboxgrid2 . It includes fix for that Horizontal scrollbar bug, also some other fixes, a new XL viewport and .hidden- classes for hiding columns on different viewports. What are your thoughts?

[roylee0704]

Thanks for the PR! @Kamahl19 I've merged and released :)