bumping default version of splunk from 6.0.3 to 6.1.3

@jtimberman Sir, I think to upgrade our splunk servers we'll want to run the chef-splunk::upgrade recipe per usual, and before removing upgrade.rb we'll want to make sure to update chef-splunk/attributes/default.rb to bump up all the splunk software urls to the desired version.  Thats what this PR is for.  I'm curious after merging this, should I tag 1.2.1 and update our pinnings, or should I re-apply the tag for 1.2.0?

Optionally we could keep chef-splunk at 6.0.3 and add these urls to our role:splunk-server and so forth as override attributes, but I figure we do want to pull in updates on the base chef-splunk.

Mostly want to confirm the tag thing, then I'll go merge and deploy this to preprod.

Probably will also want to re-run the kitchen server/client tests today as well (I did test all the updated urls to confirm they don't 404).  Think I got em right.