I just hit the exact same error. I looked at the gem details at https://rubygems.org/gems/therubyracer and it seems we were "lucky" enough to bundle a new version on the release day.
This appears to already be reported: https://github.com/cowboyd/therubyracer/issues/215
So, I figured the easiest thing is to use the last version of the gem. So I updated my Gemfile to point to the last version like so:
gem "therubyracer", "~> 0.10.2"
and I was able to successfully install the gem.
If you want to manually install the gem, you can use:
gem install therubyracer --version "~> 0.10.2"