Use ruby-1.9 hash style

Changes copied from flinc-chef/newrelic-ng@d5aa3fa
Update README to indicate Chef 11 or later is required

Rather important
Add header to plugin-agent attributes to correctly specify license
Fix problem with newrelic-plugin-agent service

Lifted directly from flinc-chef/newrelic-ng@ffea9bf
Bump version to 0.6.2

Gotta keep Berkshelf happy