Target new v8 from chromium 38

This fixes compiling errors for node-webkit v0.11.0 and atom-shell v0.18.0 as reported in #365 

`v8::Object::Set` in recent versions of chromium (v38) has no overloaded version which takes 3 arguments (as before).
I replaced those occurences with `v8::Object::ForceSet` which can take the 3rd arg `PropertyAttribute`.

Travis tests pass, except for the node-webkit-v0.11.0-ia32 one.
I tested it with atom-shell v0.19.4 on ia32 windows and it works. (That's my use-case. Couldn't upgrade past 0.17.2 before.)

This is definitely not my field of expertise, so please feel free to call bullsh**t on this.
@Mithgol your opinion?