Fix apt-key error

[grokcode]

Hello,

I was running into an error on Ubuntu:

Error: apt-key adv --keyserver hkp://subkeys.pgp.net --recv-keys 548C16BF returned 2 instead of one of [0]
Error: /Stage[main]/Newrelic::Repo/Exec[newrelic-add-apt-key]/returns: change from notrun to 0 failed: apt-key adv --keyserver hkp://subkeys.pgp.net --recv-keys 548C16BF returned 2 instead of one of [0]

These changes should fix that.

I also ended up having to change the sequence of the commands and enforce ordering with require in order to work around a Ubuntu 16.04 bug described here and here.

I believe #11 solves this same issue, however I didn't see anywhere that specifies what version of the apt module it needs, and I wasn't able to get it working with the versions I tried.