Move to a newer release of Erlang (19.0+)

[wohali]

This will help take advantage of features such as allocation of binaries not off the process heap.

The quickest way to do this is to move from jessie to stretch in the 2.x/dev Dockerfiles, to get 19.2+, which is new enough. It'd be nice to go down the route of using the Erlang Solutions packages, but that will kill attempts to do things like #67 #68 and #85 since they don't provide those platforms pre-built.

kerl is an option if we want something newer, I guess, at the expense of a longer image build time.