Fix the "datasette now publish ... --alias=x" option

[simonw]

Now have deprecated the mechanism we were using for this - running now alias without any parameters - in favour of something new:

https://zeit.co/blog/automatic-aliasing

[simonw]

I also need to fix my various .travis.yml configs to adapt to this new change.

[simonw]

This is the publish code that needs updating:

datasette/datasette/publish/now.py

Lines 79 to 96 in e7b31ae

	now_json = {"version": 1}
	if alias:
	now_json["alias"] = alias
	open("now.json", "w").write(json.dumps(now_json))
	args = []
	if force:
	args.append("--force")
	if token:
	args.append("--token={}".format(token))
	if args:
	call(["now"] + args)
	else:
	call("now")
	if alias:
	alias_args = ["alias"]
	if token:
	alias_args.append("--token={}".format(token))
	call(["now"] + alias_args)

[simonw]

This needs doing before the next release because it is interfering with my Travis code that actually runs the release: https://travis-ci.org/simonw/datasette/jobs/530597261