Fixed server URL for centos/redhat

Julian,

I found the problem with the CentOS URLs for couchbase server.  It was really a simple format thing.   Once I took our old hacked version of this cookbook out of the picture, it was clear that it wasn't the hacking that had been done to our version, but the main version that had some issues.

So, this small change gets the centos version URL right, and I'm following up with Matt Ingenthron in this message (https://forums.couchbase.com/t/rpm-location-change-breaks-couchbase-server-2-2-0-download-for-centos-cookbook/1921/3) on the Couchbase forums.

I've asked about whether they even have different packages for ubuntu, centos and debian, and will update the recipe accordingly if I get an answer.

Mean time, here's a pull req for this small change.

Also, you said that you'd run the test-kitchen tests on this version and it worked...did that include centos?  How much of the server recipe is covered by the test to which you refer?

Thanks.

Eric