Treat build number as number

I changed it to be a Number along with Job's number, but it's wrong -
Build's number should be treated as an integer (to not screw up
ordering) and Job's number can be treated as a string (because it has a
format "\d.\d")

assets/scripts/app/models/build.coffee

@@ -5,7 +5,7 @@ require 'travis/model'
   commitId:         Ember.attr('number')
 
   state:            Ember.attr('string')
-  number:           Ember.attr('string')
+  number:           Ember.attr(Number)
   branch:           Ember.attr('string')
   message:          Ember.attr('string')
   _duration:        Ember.attr(Number, key: 'duration')