Depend on ruby1.9.1-dev instead of ruby-dev which could be provided by another version of Ruby

Without this installation failed while installing json:

ERROR:  Error installing json:
	ERROR: Failed to build gem native extension.

        /usr/bin/ruby1.9.1 extconf.rb
/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>'


Gem files will remain installed in /var/lib/gems/1.9.1/gems/json-1.8.0 for inspection.
Results logged to /var/lib/gems/1.9.1/gems/json-1.8.0/ext/json/ext/generator/gem_make.out