Restore 1.9 compat, drop 1.8 compat

[vemv]

Closes https://github.com/clojure-emacs/cider-nrepl/issues/836

1.8 compat, while it was intended to be, was broken for a possibly very long time.

The reason being, the CI matrix as introduced in 1016f61754c5bcf58ed745a88620381f4afc749d was broken: CircleCI set VERSION, while the Makefile expected CLOJURE_VERSION.

So, surely, progressively factual support degraded.

This commits fixes 1.9 compatibility and ensures that the CI matrix runs as intended.

I deem 1.8 not worth fixing, given that:

• it was broken already;
• nobody complained;
• we planned to drop it; and most importantly
• it was hard to restore, since it would mean running an older Leiningen (Lein has used 1.9+ features for over 5 years, which means that the CCI images would have to be replaced with something else)
  • In particular, I was hitting Unable to resolve var: *print-namespace-maps* in this context, compiling:(leiningen/core/main.clj:342:5) while running the cider-nrepl.plugin-test.

[bbatsov]

nobody complained;

Can't argue with this argument. 😆 Thanks for tackling this!