update RVM: use .ruby-version and .ruby-gemset

Replace deprecated .rvmrc file with .ruby-version and
.ruby-gemset

Update RVM to latest stable version:

  $ rvm get stable
add Heat Transfer Basics Energy2D models

This content is imported from but does not include the
energy3d examples.

  http://energy.concord.org/htb/

The html files have been re-written to reference the new
location for the energy2d.jar archive.
import Energy2D Heat Transfer Basics
also process non-htb energy2d content w/kramdown
update generated energy2d interactives