switch to using the karma binary in node_modules/.bin

fixes CNVS-12918

Change-Id: If1abbcdb84d26a1a30d60030bcf490bb35f2a9ec
Reviewed-on: https://gerrit.instructure.com/34493
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Florence <ryanf@instructure.com>
Product-Review: Ryan Florence <ryanf@instructure.com>
QA-Review: Ryan Florence <ryanf@instructure.com>

lib/tasks/js.rake

@@ -15,7 +15,7 @@ namespace :js do
       end
     end
     Rake::Task['js:generate_runner'].invoke
-    exec("karma start --browsers #{browsers}")
+    exec("node_modules/.bin/karma start --browsers #{browsers}")
   end
 
   def matcher_for_ember_app app_name