Update HHVM_API_VERSION

Summary: Since 3.0 `Extension` class has been updated, so we need
to update HHVM_API_VERSION, or HHVM could load extensions
compiled against HHVM2.2 and crash.

Closes #3032
Closes #3090

Reviewed By: @ptarjan

Differential Revision: D1422384

Pulled By: @fredemmott