[ticket/12974] Update nightly build version to 3.2.0-a1-dev

PHPBB3-12974
Merge pull request #2871 from nickvergessen/ticket/12974

[ticket/12974] Update nightly build version to 3.2.0-a1-dev

* nickvergessen/ticket/12974:
  [ticket/12974] Update nightly build version to 3.2.0-a1-dev