Use modulo instead of cycle to split language menu

The jekyll {% cycle %} code is a little hard to understand, easy to break, and hard to update each time we need to change the number of languages displayed per column (so we don't end with a single language in the last column).

By using modulo and divided_by, ~~we would only need to edit "modulo:11" to set a different number of languages per column~~ we can automate the number of languages per column, and edit "divided_by: 2" to set a different number of columns. This seems a more elegant solution to me.

The modulo filter has been added to liquid in 2011 and seems supported even by old jekyll versions:
https://github.com/Shopify/liquid/commit/c4d713b6bbb1a61f65583de32112756200231d5e