[DRE-maint] Bug#668472: ruby-msgpack: FTBFS: can't load json when testing against ruby1.8

[Aaron M. Ucko]

Source: ruby-msgpack
Version: 0.4.6-2
Severity: serious
Justification: fails to build from source

Builds of ruby-msgpack in minimal environments (such as on the
autobuilders) have been failing because json.rb isn't part of the
standard library in 1.8.x, just 1.9.x:

/usr/bin/ruby1.8 -I/usr/lib/ruby/vendor_ruby /usr/lib/ruby/vendor_ruby/gem2deb/test_runner.rb
/usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in `gem_original_require': no such file to load -- json (LoadError)
	from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in `require'
	from ./test/test_cases.rb:9
	from -e:1:in `require'
	from -e:1
	from -e:1:in `each'
	from -e:1
ERROR: Test "ruby1.8" failed. Exiting.

Could you please add a build dependency on ruby-json to compensate,
and check with pbuilder or the like that no others are missing?

Thanks!