Prepare for a v3 release

[weierophinney]

This patch prepares for a v3 release, as the code in zend-stdlib is stable at this time. Specifically, it does the following:

• Updates the PHP dependency to ^5.5 || ^7.0.
• Removes dependencies:
  • zend-config: this was used for one test case in ArrayUtils, and was redundant, as another test case covered the same functionality, via SPL artifacts.
  • zend-serializer: this was used previously in the zend-hydrator tests, and is no longer required.
• Documentation:
  • Adds the documentation build chain introduced with Expressive and zend-servicemanager.
  • Removes the hydrator documentation (that will go under zend-hydrator).
  • Adds a migration chapter.

[gianarb]

Without dependencies!! it seems a dream! 👍

[weierophinney]

@ezimuel Once this is merged and we tag 3.0, this will VASTLY simplify the servicemanager migration effort!