Falling back to main branch instead of displaying docs for latest release

[shadowspawn]

Found via report that unreleased code showing in docs: yargs/yargs#2569 (comment)

Both the old jsonp sites are defunct, and falling through to main branch:

yargs.github.io/_layouts/docs.html

Lines 31 to 61 in 4114b79

	// use a service to wrap registry json in jsonp
	var svcs = ['https://json2jsonp.com/', 'https://jsonp.afeld.me/']
	var i = 0
	var nextSvc = function () {
	var svc = svcs[i++]
	if (!svc) {
	// no jsonp services worked, just use master
	console.log('falling back to master branch :(')
	return Flatdoc.run({
	fetcher: Flatdoc.github('yargs/yargs', 'docs/api.md')
	})
	}
	$.ajax(svc, {
	data: {
	url: 'https://registry.npmjs.org/yargs'
	},
	dataType: 'jsonp',
	timeout: 2000,
	jsonpCallback: svc.replace(/\W/g, ''),
	success: function (data) {
	var version = data['dist-tags'] && data['dist-tags'].latest
	if (!version) return nextSvc()
	console.log('determined latest version as %s via %s', version, svc)
	Flatdoc.run({
	fetcher: Flatdoc.file('https://raw.githubusercontent.com/yargs/yargs/v' + version + '/docs/api.md')
	})
	},
	error: function () {
	console.log('service failed: %s, trying next', svc)
	nextSvc()
	}

From console when loading docs:

service failed: https://json2jsonp.com/, trying next
TypeError: httpsjson2jsonpcom is not a function
[HUGE error omitted]
service failed: https://jsonp.afeld.me/, trying next
falling back to master branch :(

The docs are also slow to load, perhaps related to the failed attempts.