Encountered problem when installing sensu-plugin

    $ gem install sensu-plugin --no-rdoc --no-ri
    /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>'

This is the solution: use the ruby runtime embedded in sensu

    $ /opt/sensu/embedded/bin/gem install sensu-plugin
    Fetching: sensu-plugin-1.1.0.gem (100%)
    ERROR:  While executing gem ... (Gem::FilePermissionError)
        You don't have write permissions for the /opt/sensu/embedded/lib/ruby/gems/2.0.0 directory.
    vagrant@default-ubuntu-1404:~$ sudo /opt/sensu/embedded/bin/gem install sensu-plugin
    Successfully installed sensu-plugin-1.1.0
    Parsing documentation for sensu-plugin-1.1.0
    Installing ri documentation for sensu-plugin-1.1.0
    1 gem installed