Update README with v3.1.1 info
Update README with v3.1.1 info

piwik/piwik-sdk-ios,piwik/piwik-sdk-ios,wedrive-core/piwik-sdk-ios,piwik/piwik-sdk-ios,fireflyexperience/piwik-sdk-ios,artcom/piwik-sdk-ios,collectivehealth/piwik-sdk-ios