Overrides the dd-handler recipe to use custom gem

As per https://github.com/Shopify/ops/issues/904
Gem updates https://github.com/Shopify/chef-handler-datadog/pull/1

We need additional roles reported from our servers, so we need to customize the chef-handler-datadog gem and install it. This swaps the dd-handler recipe from the datadog cookbook for a slightly modified version of our own that installs our version of the chef-handler-datadog gem

Concerns that I have at this point, 

* I don't think its idempotent, it will probably try to install every time it runs
* version is hardcoded

Please take a look @wisq @coosh @kvs 

FYI, when I created this the Cheffile was configured to use the `additional-plugins` branch. Therefore I forked and created the PR from that branch  (i.e. there are more changes in relation to master than are shown here)