Update the phantom.js test runner to allow multiple JS injections

This allows testing via phantomjs when you need to include one or more external JS files before the compiled ClojureScript code from your project.

For example, if you wanted to include jQuery on the generated page, you could do something like this in your lein project:

```clojure
:cljsbuild {:builds [{:source-paths ["src" "test"]
                      :compiler {:output-to "target/cljs/testable.js"
                                 :optimizations :whitespace
                                 :pretty-print true}}]
            :test-commands {"unit-tests" ["runners/phantomjs.js" "resources/public/js/jquery.js" "target/cljs/testable.js"]}}
```