Use v8::Isolate version of v8::Context::New

The v8 library doesn't define the `v8::Context::New` function any longer, unless the library is compiled with `V8_USE_UNSAFE_HANDLES` ... which it isn't anymore by default.

This patch switches to the `v8::Context::New` methods with `v8::Isolate` passing.

In combination with the define from issue #19 v8js compiles with version 3.19.18 and passes the test suite.

cheers,
  stesie

NB: I wonder whether to just define `V8_USE_UNSAFE_HANDLES` in `php_v8js_macros.h` for the time being and hold on ...