Support leiningen versions > 2.8.1

[cursive-ghost]

Currently the list of supported leiningen versions is hard-coded into Cursive. This is because the integration is very brittle, and I like to test them when I allow new lein versions to be used. Unfortunately recent versions of e.g. cider/piggieback require the new nrepl and have removed support for the older clojure.tools.nrepl. The current latest leiningen version supported by Cursive (2.8.1) doesn't support the new nrepl.

[cursive-ghost]

I've added versions up to 2.9.0. I haven't tested them very thoroughly yet, but I'll get them out in an EAP release so that users can play around with them and report problems.