Travis: base linux build on jammy (22.04), macOS build on 12.1

Default linux release is 16.04 (https://docs.travis-ci.com/user/reference/linux),
which is not compatible with Node.js 18.x.x (current stable version).

Default macOS release is 10.13 (https://docs.travis-ci.com/user/reference/osx),
which is not compatible with Node.js 18.x.x (current stable version).

https://nodejs.org/en/blog/release/v18.0.0#header-toolchain-and-compiler-upgrades.

Author: chrisveness

.travis.yml

@@ -9,6 +9,8 @@ os:
 - linux
 - osx
 # windows - removed 'cos travis have made non-backward-compatible move to nvs on windows (travis-ci.community/t/12393) - sigh!
+dist: jammy          # as of Sep 2022 travis defaults Linux to xenial, which is incompatible with node.js 18.x.x
+osx_image: xcode13.2 # as of Sep 2022 travis defaults macOS to 10.13, which is incompatible with node.js 18.x.x
 
 after_success:
 - c8 -r text-lcov npm test | coveralls