[ticket/11880] Break up schema changes in the migrator

PHPBB3-11880
[prep-release-3.1.0-a3] Update version to 3.1.0-a3
[prep-release-3.1.0-a3] Add migration for 3.1.0-a3
[prep-release-3.1.0-a3] Prepare Changelog
[ticket/11880] Move get_schema_steps function to a migrator helper class

PHPBB3-11880
[prep-release-3.1.0-a3] Commit changelog for 3.1.0-a3
[ticket/11880] migration helper file

PHPBB3-11880
Merge pull request #1995 from EXreaction/ticket/11880

Schema changes can take too long and cause a timeout
[prep-release-3.1.0-a3] Update version to 3.1.0-a4-dev for future development
Merge branch 'prep-release-3.1.0-a3' into develop
[ticket/12141] Disable opcache on travis for php 5.5.8 aswell

PHPBB3-12141
Merge pull request #2017 from nickvergessen/ticket/12141

[ticket/12141] Disable opcache on travis for php 5.5.8 aswell

* nickvergessen/ticket/12141:
  [ticket/12141] Disable opcache on travis for php 5.5.8 aswell