[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/11880] Break up schema changes in the migrator

PHPBB3-11880
[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
Merge pull request #2001 from bantu/ticket/12161

[ticket/12161] Do not use subdirectories in build/save directory.
[ticket/12162] Set language to C before running diff to ensure its english

PHPBB3-12162
Merge remote-tracking branch 'github-nickvergessen/ticket/12162' into develop-olympus

* github-nickvergessen/ticket/12162:
  [ticket/12162] Set language to C before running diff to ensure its english
Merge branch 'develop-olympus' into develop

* develop-olympus:
  [ticket/12162] Set language to C before running diff to ensure its english
  [ticket/11509] Be exact when referencing the branch name.
  [ticket/12161] Do not use subdirectories in build/save directory.
  [ticket/11509] Run commit message checker only in one environment.
  [ticket/11509] Contrary to docs, COMMIT_RANGE is not available in PR mode.

Conflicts:
	.travis.yml
[develop] Remove left over conflict marker from merge 1a31f03
[ticket/12163] Use posts_per_page to calculate current page in viewtopic.php

PHPBB3-12163