Add ruby-dev to be installed whilst provisioning process.

[zylad]

At the moment puppet is failing while trying to install `sensu-plugin' gem on both server and client with the following error:

==> sensu-client: Error: Execution of '/usr/bin/gem install --no-rdoc --no-ri sensu-plugin' returned 1: Building native extensions.  This could take a while...
==> sensu-client: ERROR:  Error installing sensu-plugin:
==> sensu-client:   ERROR: Failed to build gem native extension.
==> sensu-client:
==> sensu-client:         /usr/bin/ruby1.9.1 extconf.rb
==> sensu-client: /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require': cannot load such file -- mkmf (LoadError)
==> sensu-client:   from /usr/lib/ruby/1.9.1/rubygems/custom_require.rb:36:in `require'
==> sensu-client:   from extconf.rb:1:in `<main>'
==> sensu-client:
==> sensu-client:
==> sensu-client: Gem files will remain installed in /var/lib/gems/1.9.1/gems/json-1.8.3 for inspection.
==> sensu-client: Results logged to /var/lib/gems/1.9.1/gems/json-1.8.3/ext/json/ext/generator/gem_make.out
==> sensu-client: Error: /Stage[main]/Sensu::Package/Package[sensu-plugin]/ensure: change from absent to present failed: Execution of '/usr/bin/gem install --no-rdoc --no-ri sensu-plugin' returned 1: Building native extensions.  This could take a while...

This PR should solve it.