Change default PHP 'agent_action' to upgrade

Using 'install', the version is inherited from the previous resource of the same name, one we just tried to remove:

```ruby
package 'newrelic-php5' do
  action :remove
  version '3.0.5.95'
end

# install/update latest php agent
package 'newrelic-php5' do
  action node['newrelic']['php-agent']['agent_action']
  notifies :run, 'execute[newrelic-install]', :immediately
end
```

By setting `node['newrelic']['php-agent']['agent_action']` to 'upgrade' Chef will install the latest version of the agent package.