升级多次，最终无果
一直死在
Calculating the changes
Could not calculate the upgrade
An unresolvable problem occurred while calculating the upgrade.
This can be caused by:
* Upgrading to a pre-release version of Ubuntu
* Running the current pre-release version of Ubuntu
* Unofficial software packages not provided by Ubuntu
If none of this applies, then please report this bug using the
command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal.
Restoring original system state
按照网上的教程删除gnuplot ：sudo apt-get purge gnuplot*
最终还是不行
后来查看 /var/log/dist-upgrade/apt.log
查看是什么软件导致无法升级，
最终我的是因为 ros-indigo-* 机器人系统的原因
因此删除软件 ：sudo apt-get remove ros-indigo-*
重新开始进行升级就好了
sudo do-release-upgrade
