Adapt the test site script to simple API's naming change of SimpleRWC to SimpleReadiumJs.

This change in commit b91cc8a197f1dc46bfc86182906211eb8e304773 has broken the test site:

```diff
index 6b31366..479782a 100644
--- epub-modules/simple-readium-js/simple_rwc_template.js.erb
+++ epub-modules/simple-readium-js/simple_rwc_template.js.erb
@@ -1,6 +1,6 @@
 <%= epub_reading_system %>
 
-var SimpleRWC = function (elementToBindReaderTo, viewerPreferences, packageDocumentURL, packageDocumentXML, renderStrategy) {
+var SimpleReadiumJs = function (elementToBindReaderTo, viewerPreferences, packageDocumentURL, packageDocumentXML, renderStrategy) {
     
     // Epub modules
     var EpubCFI = {};
```

This pull request fixes `showEpub.js` to use the new name.