Use ruby1.9.1-dev in quantal.

Ruby 1.9.x is the default and when installing ruby1.8-dev, one gets the error:

/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in 'require': cannot load such file -- mkmf (LoadError)
  	from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in 'require'
  	from extconf.rb:1:in '<main>'
  rake aborted!

So the ruby1.8-dev files are not picked up and useless for our case.

Signed-off-by: Peter Soetens <peter@thesourceworks.com>