Log `yarn install` output while it is running

This change makes it so we don’t wait until it
Exits to log the output. Because something is going
On in the deploy boxes where it just dies midway and 
never finishes.

Test plan:
* run rake canvas: compile_assets, 
* you should see log output while the yarn part runs

Change-Id: I8e2fc0cc559e631bbde2372fa548e93d77af13a6
Reviewed-on: https://gerrit.instructure.com/130115
Tested-by: Jenkins
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>

lib/tasks/js.rake

@@ -50,9 +50,8 @@ namespace :js do
   desc "Ensure up-to-date node environment"
   task :yarn_install do
     puts "node is: #{`node -v`.strip} (#{`which node`.strip})"
-    output = `yarn install --frozen-lockfile`
+    system "yarn install --frozen-lockfile"
     unless $?.success?
-      puts output
       raise 'error running yarn install'
     end
   end