bump the therubyracer and libv8 gems

the newer versions fix compilation issues for OS X 10.9 Mavericks
(https://github.com/cowboyd/therubyracer/issues/261)
Merge pull request #339 from alphagov/bump-therubyracer-libv8

bump the therubyracer and libv8 gems
Revert "pull request #339 from alphagov/bump-therubyracer-libv8"

This reverts commit 4f594c190577aca90e190f8469db84f162f39281, reversing
changes made to 1e41dbabf5eb49c8f522c70b7ea7d9830ed81904.