Fix issue with prerendering; add boot.core dep

* Add a `:dependencies` entry for `boot.core` because even though it's
  an implicit dependency that is already present in the class path there
  are still compatibility issues that may need to be debugged. Stating
  the dependency explicitly in `project.clj` will help there.

* The prerender task was adding extra <script> tags to the document body
  when prerendering because the `main.js` file was not in the same
  staging directory as the `index.html` file.