Rackup has moved to it's own gem

[stbenjam]

In rack/rack#1937, rackup was moved to a different gem, so the message from the assemble script is incorrect.

The error says:

s2i-ruby-container/2.2/s2i/bin/assemble

Lines 52 to 55 in ab1ca4b

	if ! bundle exec rackup -h &>/dev/null; then
	echo "WARNING: Rubygem Rack is not installed in the present image."
	echo " Add rack to your Gemfile in order to start the web server."
	fi

This should say rackup if rack is >= 3.x

[phracek]

@stbenjam Can you please provide pull request that fixes this issue, as you are expert?

[stbenjam]

I'm not an s2i expert so I don't even know how I'd begin to test my change.