[ticket/12316] Add Build Status Icon for develop-ascraeus to README.md

PHPBB3-12316
Merge remote-tracking branch 'VSEphpbb/ticket/12316' into develop-olympus

* VSEphpbb/ticket/12316:
  [ticket/12316] Add Build Status Icon for develop-ascraeus to README.md
[ticket/12318] Correctly setup HHVM for functional tests.

PHPBB3-12318
[ticket/12318] Upgrade to a recent stable version of HHVM.

PHPBB3-12318
[ticket/12318] Add PPA providing dependencies for recent HHVM on Ubuntu 12.04.

PHPBB3-12318
[ticket/12318] Always install new config file on HHVM upgrade conflict.

PHPBB3-12318
[ticket/12318] Pin HHVM version to 3.0.0~precise.

PHPBB3-12318
[ticket/12318] Use MySQL instead of MySQLi for now on HHVM.

PHPBB3-12318
Merge remote-tracking branch 'bantu/ticket/12318-olympus' into develop-olympus

* bantu/ticket/12318-olympus:
  [ticket/12318] Use MySQL instead of MySQLi for now on HHVM.
  [ticket/12318] Pin HHVM version to 3.0.0~precise.
  [ticket/12318] Always install new config file on HHVM upgrade conflict.
  [ticket/12318] Add PPA providing dependencies for recent HHVM on Ubuntu 12.04.
  [ticket/12318] Upgrade to a recent stable version of HHVM.
  [ticket/12318] Correctly setup HHVM for functional tests.
[ticket/12341] Add tests for get_username_string()

PHPBB3-12341
Merge remote-tracking branch 'Elsensee/ticket/12341' into develop-olympus

* Elsensee/ticket/12341:
  [ticket/12341] Add tests for get_username_string()
[ticket/10851] Set disallowed content to empty array if checking is disabled

The disallowed content defaults to a standard set of mimetype triggers by
default. If one doesn't want to check the attachments mimetype triggers then
we need to set the disallowed content to an empty array.

PHPBB3-10851
[ticket/12318] Revert "Pin HHVM version to 3.0.0~precise."

This reverts commit f6614f2078740675ca0228b348500991888c2140 as the install
command started failing with "Version '3.0.0~precise' for 'hhvm' was not found"
in the meantime.

PHPBB3-12318
Merge remote-tracking branch 'bantu/ticket/12318' into develop-olympus

* bantu/ticket/12318:
  [ticket/12318] Revert "Pin HHVM version to 3.0.0~precise."
[ticket/12353] Fix displayed attachments in ACP

PHPBB3-12353
Merge remote-tracking branch 'Elsensee/ticket/12353' into develop-olympus

* Elsensee/ticket/12353:
  [ticket/12353] Fix displayed attachments in ACP
Merge remote-tracking branch 'Marc/ticket/10851' into develop-olympus

* Marc/ticket/10851:
  [ticket/10851] Set disallowed content to empty array if checking is disabled