update-alternatives --config gem
There are 2 choices for the alternative gem (providing /usr/bin/gem).

  Selection    Path               Priority   Status
------------------------------------------------------------
  0            /usr/bin/gem1.8     180       auto mode
  1            /usr/bin/gem1.8     180       manual mode
* 2            /usr/bin/gem1.9.1   10        manual mode
update-alternatives --config ruby
There are 2 choices for the alternative ruby (providing /usr/bin/ruby).

  Selection    Path                Priority   Status
------------------------------------------------------------
  0            /usr/bin/ruby1.8     50        auto mode
  1            /usr/bin/ruby1.8     50        manual mode
* 2            /usr/bin/ruby1.9.1   10        manual mode

ruby -v
ruby 1.9.3p0 (2011-10-30 revision 33570) [x86_64-linux]

bundle update
Fetching gem metadata from https://rubygems.org/.........
Fetching gem metadata from https://rubygems.org/..
Resolving dependencies...
Using ffi (1.9.0)
Using mime-types (1.25)
Using ethon (0.6.1)
Using json (1.8.1)
Using mini_portile (0.5.1)
Installing nokogiri (1.6.0)
Gem::InstallError: nokogiri requires Ruby version >= 1.9.2.
An error occurred while installing nokogiri (1.6.0), and Bundler cannot continue.
Make sure that `gem install nokogiri -v '1.6.0'` succeeds before bundling.