Bump minimum required Erlang version to 19.2

[michaelklishin]

This is a follow-up to #546.

Next releases of our key dependencies in master — Elixir, Cowboy, possibly Ranch — won't support OTP 18. We occasionally run into standard library functions not being available OTP 18, which requires post-factum fixes.

Therefore we are considering requiring 19.2 for 3.7.0 while we can. It seems to be the wise thing to do.

As with any minimum version requirement, main downside is the ease of provisioning. OTP 19.2 is only available in Ubuntu 17.04, but Erlang Solutions repositories, Debian Stretch and Debian Jessie backports repositories provide 19.2 or later versions. The percentage of users who use Erlang Solutions
repositories or Debian backports is already quite high (this is a non-scientific claim, we don't collect any data about RabbitMQ installations). Ubuntu 17.10, which is not too far off, will include OTP 20.

To make sure by the time 3.8.0 reaches RC stage we don't tie ourselves to Elixir, Ranch or Cowboy versions from 1 year ago — which may or may not be getting any patch releases, by the way — the team decided that bumping the minimum required version to 19.2 is worth it.