[ticket/11996] Add explanation how to correctly remove and update an extension

See http://area51.phpbb.com/phpBB/viewtopic.php?f=81&t=44001 for more
information about the decision.

PHPBB3-11996
[ticket/11996] Use a table for better styling

PHPBB3-11996
[ticket/11985] Reorganise travis/install-php-extensions.sh

PHPBB3-11985
[ticket/11985] Enable APC on command line so it can be used by PHPUnit.

PHPBB3-11985
[ticket/11985] Don't try to install APC on PHP 5.5 or higher.

PHPBB3-11985
[ticket/11996] Remove note until we really do disable the board

PHPBB3-11996
Merge pull request #1942 from nickvergessen/ticket/11996

[ticket/11996] Add explanation how to correctly remove and update an extension
[ticket/11880] Break up schema changes in the migrator

PHPBB3-11880
[ticket/12159] Fix codesniffer complaints due to profilefields and passwords

The two PRs regarding profilefields and passwords manager that were recently
merged caused a few codesniffer complaints that are fixed by these changes.

PHPBB3-12159
Merge pull request #1997 from marc1706/ticket/12159

[ticket/12159] Fix codesniffer complaints due to profilefields and passwords

* marc1706/ticket/12159:
  [ticket/12159] Fix codesniffer complaints due to profilefields and passwords
[ticket/11509] Contrary to docs, COMMIT_RANGE is not available in PR mode.

PHPBB3-11509
[ticket/11509] Run commit message checker only in one environment.

PHPBB3-11509
[ticket/11985] Rename install -> setup. This is more generic and consistent.

PHPBB3-11985
[ticket/11985] Do not try to setup PHP extensions on HHVM.

PHPBB3-11985
[ticket/12161] Do not use subdirectories in build/save directory.

This is pretty much pointless because a new build will wipe the save directory
and there are only a few files in there anyway.

PHPBB3-12161
[ticket/12141] PHP extension setup must happen before webserver + PHP-FPM.

PHPBB3-12141
[ticket/12141] Disable broken opcache extension on PHP 5.5.7.

PHPBB3-12141
[ticket/11509] Be exact when referencing the branch name.

PHPBB3-11509
Merge pull request #1998 from bantu/ticket/12141

[ticket/12141] Disable opcache extension on PHP 5.5.7
Merge pull request #1999 from bantu/ticket/11509

[ticket/11509] Further improvements/fixes for the Travis CI commit message checker