String.join was new for Java 8. Since minimum requirement is Java 6, use a different static method.
Making sure test fails if cat is not called.

The other test on cat-files did not check for this. Obviously essential for making the cat files work.
Merge branch 'solution'

Conflicts:
	test/clojure_workshop/cat_test.clj
	test/clojure_workshop/ex_2.clj