Drop support for less 2

Roe now supports less 3 only, as of #337

If upgrading you should check that any less compiling dependencies support less 3, and that any of the following are updated:

Essentially less will no longer require calc() calls to be wrapped in ~'', and back-ticks (for inline javascript) are no longer enabled by default (and ideally should be removed).

This also contains a small dependency audit: #338