SitemapTreemaker can't be both a class and a module + Travis CI

* `SitemapTreemaker` cannot be both a class and a module with more modern rubies. Moved `SitemapTreemaker` into `SitemapTreemaker::Sitemap`.
* Updated default .rvmrc version.
* Test that the sitemap actually works, if you returned any valid xml it would pass tests before. Now finding a well known node in a fixture.
* Actually use fakeweb in tests.
* Added Travis-CI, you should enable it on travis-ci.org before merging this. Successful build in https://travis-ci.org/dblock/sitemap_treemaker/builds/23929204.