Fix: Dropbox changed login form action URL

Dropbox has changed the login URL, leading to a 404 for the old one.

Fixes jakajancar/DropboxUploader#37
Version bump v1.1.18

- Fix Login unsuccessful. See Issue jakajancar/DropboxUploader#37
Merge pull request #39 from hakre/master

Update to Update to 1.1.18