Change `task.invoke` to `Rake::Task.invoke`

Using `task()` reopens the definition of the task, and all we really
need to do is get a reference to the task itself to invoke it.

rakefiles/assets.rake

@@ -147,7 +147,7 @@ end
         # env arguments to be passed to all dependent tasks.
         desc "Compile coffeescript and sass, and then run collectstatic in the specified environment"
         task "#{system}:gather_assets:#{env}" do
-          task(:gather_assets).invoke(system, env)
+          Rake::Task[:gather_assets].invoke(system, env)
         end
     end
 end

rakefiles/django.rake

@@ -30,7 +30,7 @@ end
         desc
     task system, [:env, :options] do |t, args|
         args.with_defaults(:env => 'dev', :options => default_options[system])
-        task(:runserver).invoke(system, args.env, args.options)
+        Rake::Task[:runserver].invoke(system, args.env, args.options)
     end
 
     desc "Start #{system} Celery worker"

rakefiles/jasmine.rake

@@ -104,12 +104,12 @@ end
 [:lms, :cms].each do |system|
     desc "Open jasmine tests for #{system} in your default browser"
     task "browse_jasmine_#{system}" do
-        task(:browse_jasmine).invoke(system, 'jasmine')
+        Rake::Task[:browse_jasmine].invoke(system, 'jasmine')
     end
 
     desc "Use phantomjs to run jasmine tests for #{system} from the console"
     task "phantomjs_jasmine_#{system}" do
-        task(:phantomjs_jasmine).invoke(system, 'jasmine')
+        Rake::Task[:phantomjs_jasmine].invoke(system, 'jasmine')
     end
 end