bug (build page) using local page variable instead of currentPage

When rendering pages, we were using a local page variable from
currentPage.page. Since we didn't update this variable after calling
the 'render:pre:page' plugin stage, the page content wouldn't get
updated.

Changing this to use the currentPage.page always and getting rid
of the local page variable all together.
Merge pull request #397 from doowb/bug-render-pre-page

bug (build page) using local page variable instead of currentPage