Fix broken ckeditor paths

Hopefully fixes two issues:

1. PR #105 introduced an issue when the assets_version is set (the assets_version was getting appended to the path)

2. ckeditor javascript paths were broken when Symfony is non-root. Assetic appeared to ignore the subdirectory symfony was installed to, but removing the leading / in the output seems to have fixed that for non-root installations (I haven't tested when installed to root)