missing dependency to json ? (using ruby 1.8.7)

[ofa-]

pi@raspberrypi ~ $ gem install sonice
....
pi@raspberrypi ~ $ SONICE_VOTING=0 sonice
/var/lib/gems/1.8/gems/haml-4.0.4/lib/haml/parser.rb:86: warning: regexp has invalid interval
/usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in `gem_original_require': no such file to load -- json (LoadError)
...

pi@raspberrypi ~/so-nice $ ruby -v
ruby 1.8.7 (2012-02-08 patchlevel 358) [arm-linux-eabihf]
pi@raspberrypi ~/so-nice $ uname -a
Linux raspberrypi 3.6.11+ #538 PREEMPT Fri Aug 30 20:42:08 BST 2013 armv6l GNU/Linux

[ofa-]

anyway, installing json does not help enough:

pi@raspberrypi ~ $ gem install json
Fetching: json-1.8.1.gem (100%)
Building native extensions. This could take a while...
Successfully installed json-1.8.1
1 gem installed
Installing ri documentation for json-1.8.1...
Installing RDoc documentation for json-1.8.1...

pi@raspberrypi ~ $ SONICE_VOTING=0 sonice
/var/lib/gems/1.8/gems/haml-4.0.4/lib/haml/parser.rb:86: warning: regexp has invalid interval
/usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in `gem_original_require': /var/lib/gems/1.8/gems/sonice-2.1.0/lib/sonice/app.rb:12: syntax error, unexpected ':', expecting kEND (SyntaxError)
set :haml, format: :html5

[ofa-]

requires ruby 1.9 (tested to work on ruby 1.9.1)