Unable to start scout_realtime

[netsetter]

Just installed scout_realtime on CentOS 6.7 inclusive Ruby and RubyGems without any error messages.

When now trying to start scout_realtime, I'm stuck with following error:

/usr/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:in gem_original_require': no such file to load -- json (LoadError) from /usr/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:31:inrequire'
from /usr/lib/ruby/gems/1.8/gems/scout_realtime-1.0.5/lib/scout_realtime.rb:23
from /usr/lib/ruby/gems/1.8/gems/scout_realtime-1.0.5/bin/scout_realtime:4:in load' from /usr/lib/ruby/gems/1.8/gems/scout_realtime-1.0.5/bin/scout_realtime:4 from /usr/bin/scout_realtime:19:inload'
from /usr/bin/scout_realtime:19

Any idea what's still missing?

[natzar]

$ gem install json
$ scout_realtime