Update dropbox.js to 0.8.1

dropbox.js is already in cdnjs, and this pull request adds the latest minor release.

I got the source code from https://github.com/dropbox/dropbox-js/tree/v0.8.1

I followed the build process is at https://github.com/dropbox/dropbox-js/blob/master/doc/development.md

The following all point to the correct version number:
* git tag
* last commit message
* line 3 in package.json

Thank you very much for hosting dropbox.js!