Use chef_gem resource

The chef_gem resource was added in 0.10.9 to handle both omnibus
installs and to handle the common pattern of needing a gem within the
same Chef run.

[1]: http://wiki.opscode.com/display/chef/Resources#Resources-Differencesbetweenchef_gemandgem_packageresources
Only install ruby-dev if required

Omnibus doesn't need a ruby-dev package so only install it if we fail
our omnibus check
Skip ri and rdoc generation

Generating the docs takes forever for some gems and is just a waste of
time, why would you read ri/rdoc on your server?
chef_gem bugged when passed an options string

Filed upstream bug. As it's not critical to pass those options (just an
optimization) I've removed the buggy line for now.

http://tickets.opscode.com/browse/CHEF-3907
Merge pull request #16 from racker/use_chefgem

Use chef_gem resource