Change `last_contact` to `contacted_at`

doc/api/runners.md

@@ -116,7 +116,7 @@ Example response:
     "description": "test-1-20150125",
     "id": 6,
     "is_shared": false,
-    "last_contact": "2016-01-25T16:39:48.066Z",
+    "contacted_at": "2016-01-25T16:39:48.066Z",
     "name": null,
     "platform": null,
     "projects": [
@@ -166,7 +166,7 @@ Example response:
     "description": "test-1-20150125-test",
     "id": 6,
     "is_shared": false,
-    "last_contact": "2016-01-25T16:39:48.066Z",
+    "contacted_at": "2016-01-25T16:39:48.066Z",
     "name": null,
     "platform": null,
     "projects": [

lib/api/entities.rb

@@ -380,7 +380,7 @@ module API
     class RunnerDetails < Runner
       expose :tag_list
       expose :version, :revision, :platform, :architecture
-      expose :contacted_at, as: :last_contact
+      expose :contacted_at
       expose :token, if: lambda { |runner, options| options[:current_user].is_admin? || !runner.is_shared? }
       expose :projects, with: Entities::ForkedFromProject do |runner, options|
         if options[:current_user].is_admin?