work around missing full_url in jenkins payload

This doesn't work when there are multiple Jenkins master defined but is
good enough for now. The right way to fix this would be to have each
`Janky::Builder::Client` have its own `Janky::Builder::Receiver` with
its own `url` attribute.

/cc #66 #76

lib/janky/builder/payload.rb

@@ -1,14 +1,18 @@
 module Janky
   module Builder
     class Payload
-      def self.parse(json)
+      def self.parse(json, base_url)
         parsed = Yajl.load(json)
         build  = parsed["build"]
 
+        full_url = build["full_url"]
+        path = build["url"]
+        build_url = full_url || "#{base_url}#{path}"
+
         new(
           build["phase"],
           build["parameters"]["JANKY_ID"],
-          build["full_url"],
+          build_url,
           build["status"]
         )
       end

lib/janky/builder/receiver.rb

@@ -3,7 +3,8 @@ module Builder
     class Receiver
       def self.call(env)
         request = Rack::Request.new(env)
-        payload = Payload.parse(request.body)
+        default_base_url = Builder[:default].url
+        payload = Payload.parse(request.body, default_base_url)
 
         if payload.started?
           Build.start(payload.id, payload.url)