Use HTTPS for images in README.md
Use HTTPS for images in README.md

This change prevents mixed-content warnings on pages displaying the readme via HTTPS.
rschiang/etherpad-lite,ether/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,cloudfoundry-community/etherpad-lite-cf,aptivate/etherpad-lite,joequant/etherpad-lite,joequant/etherpad-lite,rschiang/etherpad-lite,lpagliari/etherpad-lite,tiblu/etherpad-lite,storytouch/etherpad-lite,aptivate/etherpad-lite,ether/etherpad-lite,ether/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,lpagliari/etherpad-lite,cloudfoundry-community/etherpad-lite-cf,University-of-Potsdam-MM/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,tiblu/etherpad-lite,tiblu/etherpad-lite,rschiang/etherpad-lite,storytouch/etherpad-lite,tiblu/etherpad-lite,lpagliari/etherpad-lite,aptivate/etherpad-lite,storytouch/etherpad-lite,joequant/etherpad-lite,University-of-Potsdam-MM/etherpad-lite,lpagliari/etherpad-lite,joequant/etherpad-lite,aptivate/etherpad-lite,rschiang/etherpad-lite