解决方法
将/etc/apt/sources.list 中所有http://ppa.launchpad.net 都替换成 https://launchpad.proxy.ustclug.org.
trouble shooting
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY XXXXXXXXXXXXX
解决方法:加KEY
sudo gpg --keyserver keyserver.ubuntu.com --recv XXXXXXXXXXXXX
sudo gpg --export --armor XXXXXXXXXXXXX | sudo apt-key add -