run gem_npm tasks concurrently

npm installing for some gem should not be affected
by npm installing for another gem. so by doing them concurrently,
we can speed up the build process.

test plan:
* run `npm install`
* it should be faster because the npm modules for 
  canvas_i18nliner and selenimum install at the same time
  instead of one after the other


Change-Id: I73d18db0fd3473e929ac1abff25206a929ab3a45
Reviewed-on: https://gerrit.instructure.com/84599
Reviewed-by: John Corrigan <jcorrigan@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>
Tested-by: Jenkins
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>

script/gem_npm

@@ -3,6 +3,8 @@
 # gems that do node-y stuff
 for path in $(ls -1 gems/*/package.json); do
   cd ${path:0:${#path}-13}
-  npm $1
+  echo "running npm $1 for $path"
+  npm $1 &
   cd ../..
 done
+wait