therubyracer gem isn't compatible with Windows.

Also, I was doing some research on the gem and it looks like Heroku discourages it's use:

"If you were previously using therubyracer or therubyracer-heroku, these gems are no longer required and strongly discouraged as these gems use a very large amount of memory."

If we are deploying using Heroku, maybe we should consider using a different gem.