Upgrade libv8 dependency to 6.3 branch

[pinepain]

v8 6.3 is out and as we stick to the latest v8 version, it's time.

There is also change in late 6.2 (Pass Isolate pointer to String::Utf8Value/Value constructors v8/v8@fe59853 in 6.2.396) we might benefit from and as 6.3 is out there is no major reason against upgrading libv8 dependency.

• release libv8-6.3 to PPA
• release v8@6.3 to homebrew tap
• update php-v8 deb build and runtime requirements
• update php-v8 PPA dependency to include libv8-6.3 PPA
• update php-v8 build requirements (config.m4, .travis.yml, CMakeLists.txt, provision)

[pinepain]

After updating some tests become red and reports leaks - https://travis-ci.org/pinepain/php-v8/jobs/271257124. I'm not sure was there any changes in v8 internals that affects userland code and thus need some adjustmets or it's a v8 internals issue or it just false-positive. Filled a report to v8-users mail lists - https://groups.google.com/forum/#!topic/v8-users/qu1HrjbgRsg.

Moving this into separate issue #47.