Unspecified dependency on json gem

[dylanvaughn]

While testing I spun up a fresh ubuntu install and installed heirloom. Trying to run the heirloom command failed with:

ubuntu@ip-10-197-47-60:~$ heirloom -h
/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 /var/lib/gems/1.8/gems/heirloom-0.11.0.beta.1/lib/heirloom/cli.rb:1
    from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in `gem_original_require'
    from /usr/lib/ruby/vendor_ruby/1.8/rubygems/custom_require.rb:36:in `require'
    from /var/lib/gems/1.8/gems/heirloom-0.11.0.beta.1/bin/heirloom:4
    from /usr/local/bin/heirloom:19:in `load'
    from /usr/local/bin/heirloom:19

doing:

$ sudo gem install json

fixed it.