Mark invalid concurrent setting input properly.

This adds a CSS class if the content is invalid.

assets/scripts/app/controllers.coffee

@@ -58,10 +58,11 @@ Travis.RepoSettingsController = Em.ObjectController.extend
 
   settingsChanged: (->
     if parseInt(@get('settings.maximum_number_of_builds')) > 0
+      @set('settings.maximum_number_of_builds_valid', '')
       @get('model').saveSettings(@get('settings')).then null, ->
         Travis.flash(error: 'There was an error while saving settings. Please try again.')
     else
-      Travis.flash(error: 'The maximum number of concurrent builds needs to be a number greater than zero.')
+      @set('settings.maximum_number_of_builds_valid', 'invalid')
   ).observes('settings.maximum_number_of_builds')
 
   save: ->

assets/scripts/app/templates/repo/settings.hbs

@@ -34,7 +34,7 @@
         </div>
 
         <div class="settings-row">
-          <div class="short-settings-element">
+          <div class="short-settings-element" {{bind-attr class="settings.maximum_number_of_builds_valid"}}>
             {{input value=settings.maximum_number_of_builds size="4" pattern='/^[0-9]+$/'}}
           </div>
           <label>

assets/styles/settings.sass

@@ -21,4 +21,10 @@
     margin: 0px 15px
     display: inline-block
     
+  .invalid
+    input
+      border-width: 1px
+      border-color: red
+      background: #ffb6c1
+