The issue is because the Apple WWDRCA expired today (Feb 14, 2016):https://developer.apple.com/support/certificates/expiration/
Open Keychain Access, and in the menu, click View -> Show Expired Certificates. Then, delete the expired Apple Worldwide Developer Relations Certificate Authority from both the login andSystem Keychains. Install the renewed certificate from Apple by downloading from https://developer.apple.com/certificationauthority/AppleWWDRCA.cer and then opening it.
Also you will need to do restart xcode to reflects the changes -> Clean your project -> Build then try again to generate your archive. Ref: 