login.html script for ajax should be updated

[ej-sanmartin]

Hello!
So in my editor, while writing the signinCallback function, I got an error saying "$ not defined" meaning that the jquery script is not connecting to the file. Upon inspecting the link, I get a "your file is not found message. Upon inspecting chrome developer tools with login.html, there is an error with the script saying "Failed to load resource: net::ERR_FILE_NOT_FOUND". I'll do a pull request for updating the script link!