Standardize on `dest_` or `destination_`, but not both

[gjtorikian]

Both prefixes are currently being used:

repository-sync/lib/app.rb

Lines 41 to 51 in a9b2730

	@destination_repo = params[:dest_repo]
	halt 500, 'Missing `dest_repo` argument' if @destination_repo.nil?
	@payload = JSON.parse(payload_body)
	halt 202, "Payload was not for master, was for #{@payload['ref']}, aborting." unless master_branch?(@payload)
	@squash = params[:squash]
	# keep some important vars
	process_payload(@payload)
	@destination_hostname = params[:destination_hostname] || 'github.com'

That's silly. Pick one.