Clarify "the html element" for progress bars

When I first read "the progress bar is implemented on the html element" I thought, "What html element?" I think this change makes it a bit clearer that it's the actually the `<html>` tag
Merge pull request #417 from smidwap/patch-1

Clarify "the html element" for progress bars
Note that the progress bar will be on by default from Turbolinks 3.0
Release new version with the progress bar feature
Reflect the redirected url before actually changing the page, so page:change listeners get to see the final URL, rather than the before-redirect URL
Bump for bug fix
Merge branch 'master' into tom/srcset