If anybody is having trouble installing concrete5.7 there is a simple fix for development purposes. Concrete5.7 invalidates a session if the user agent and requestor ip are different than the previous request. c9 workspaces sit behind proxies and almost every request comes different than the previous.
To fix this and maintain session persistence you can comment out line 59 of concrete/src/Session/Session.php
59| // $session->invalidate();
Here is a gist of the working example.