Fix test in clojure ETL exercise
clojure.string/split was including an empty string as a letter

For some reason perhaps only known to Rich Hickey, `(clojure.string/split
"abc" #"")` evaluates to `["" "a" "b" "c"]` (i.e. includes an empty
string). I don't think this is what we want for the ETL test - and in fact the
current example solution does not pass the tests for this reason. I've
updated the tests to use `re-seq` instead, which gets us the letters
minus the blank space, and I've simplified the example solution a little
as it didn't need nested for loops.
Merge pull request #1187 from rsslldnphy/fix/clojure-etl

Fix test in clojure ETL exercise