Migrate Pinkerton class from runkit to UOPZ

* Rewrite Pinkerton::spyOn and Pinkerton::stopSpyingOn to use
  uopz_function and uopz_restore respectively
* Remove the Pinkerton::getSpy function as the UOPZ handler closes over
  the spy
Depend on ext-uopz and not ext-runkit
Install ext-uopz during the Travis CI build