for github you need https:// not http://
GitHub automatically redirects you to the https URL so that's definitely not the problem.
Unfortunately GitHub downloads are a constant source of trouble (but they are really handy, that's why so many developers - including me - are using them). One of the issues has been addressed in this bug report
, you could try to manually update the HTTP client as it is proposed in the last comment of the bug report. Another problem might be that there is something wrong with SSL support in PHP or that you are using a proxy, SSL support for proxies has only been added to DokuWiki a couple of weeks ago.
Instead of using the plugin manager you could also simply follow the instructions for manual plugin installation
(it's as easy as downloading, unzipping and renaming the plugin directory).