$ gem help commands
will show you your options.
You can uninstall the rails gem and dependencies of: actionmailer,
actionpack, actionwebservice, activerecord, and activesupport.
$ sudo gem uninstall <gem-name>
There might be a faster one-line but I don't know it off the top of my
head. Then just use the version flag when installing rails 1.1.6.
$ gem help install
will show you the options.
Hope this helps.