update to libv8-6.x ilne

[blshkv]

I'm having a problem compiling the required REALLY OLD libv8-3.16.14.15, see the bug here:
The bugreport is filled in the link below:
rubyjs/libv8#264

The bug must be somewhere in the v8 library, but it is very unlikely that somebody will be willing to look at the old version.

So I suggest to move on, and migrate to version 6.x

[TigerWolf]

See this: #348

[lloeki]

Sorry but this gem is unmaintained, so I will close this issue.

We recommend going with mini_racer + libv8-node. If you feel the issue is still relevant to either of those, please open an issue over there!