当我安装新包时，收到以下错误消息：
$ sudo apt-get install jetty
Reading package lists... Done
Building dependency tree
Reading state information... Done
jetty is already the newest version.
0 upgraded, 0 newly installed, 0 to remove and 198 not upgraded.
1 not fully installed or removed.
After this operation, 0 B of additional disk space will be used.
Do you want to continue? [Y/n] Y
Setting up ruby1.9.1 (1:1.9.3.551-2bbox1~trusty1) ...
update-alternatives: error: alternative gem.1.gz can't be slave of gem: it is a slave of ruby
dpkg: error processing package ruby1.9.1 (--configure):
subprocess installed post-installation script returned error exit status 2
Errors were encountered while processing:
ruby1.9.1
E: Sub-process /usr/bin/dpkg returned an error code (1)
我不知道该如何解决这个问题。