Call v8::RegisterExtension using unique_ptr

[jalr]

Using this fix, I was able to build v8js with v8 version 7.6.57

[stesie]

Hej jalr, thanks for your contribution. There was one issue left regarding a double-free segfault with the extension related tests. Since php-v8js still was managing the v8::Extension * instance ... however v8 was now expecting to do that on it's own (hence the unique_ptr)

I tried to address that with #415 and will merge once tests are green