Build also requires ruby1.9.1-dev

Frustratingly, the build has been failing but Jenkins has not
been noticing; 'gem install compass' fails with

/usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)

without ruby1.9.1-dev installed.

Signed-off-by: Dan Mick <dan.mick@inktank.com>