Bump therubyracer to 0.11.4

Older versions have issues installing if a newer version of libv8 is
installed (even though it depends on the older version).  See
https://github.com/cowboyd/therubyracer/issues/166 for details.
Merge pull request #275 from alphagov/bump_therubyracer

Bump therubyracer to 0.11.4