Add direct V8 support (via d3)

[ojab]

rake test passes here with v8-4.5.103.29 on Mac OS X.
It also should be added to travis, but I'm not really sure how it should be done (plain sudo apt-get install v8 in before_install or somewhat else).

[rails-bot]

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[rafaelfranca]

@ojab thanks! Could you try adding this to the travis config?

addons:
  apt:
    packages:
    - v8

[ojab]

Unfortunately there is no pkgs with d3 in ubuntu (only libv8 without console runner), so tests fail.
Added v8 installation/testing to osx instead, it works.