Fix clojurescript clojure.string dependency in core.cljx

When using `cli-firmata v2.0.0` in the TestDrive Dashboard project, I get the following warning during Clojurescript compilation.

`WARNING: Use of undeclared Var clojure.string/join at line 171 file:/Users/blakejakopovic/.m2/repository/clj-firmata/clj-firmata/2.0.0/clj-firmata-2.0.0.jar!/firmata/core.cljs`

For Clojurescript, you need to require `clojure.string` specifically before using it. I couldn't reproduce the warning during `clj-firmata` compilation, however I tested it against TestDrive, and it fixed the warning.