add node.js v15 to build matrix
add node.js v15 to build matrix

Signed-off-by: Christopher Hiller <cba4593484830aa757cb097d8d9f0ea9b692b304@boneskull.com>

adamgruber/mocha,mochajs/mocha,adamgruber/mocha,adamgruber/mocha,mochajs/mocha,mochajs/mocha