Use --prefer-source flag for composer

- so that travis will be able to build properly
Merge #5855 to develop
Fixed CS
Merge branch 'combineunionorder' of git://github.com/samsonasik/zf2 into hotfix/5855

* 'combineunionorder' of git://github.com/samsonasik/zf2:
  fix T_OBJECT_OPERATOR on php 5.3
  fix array syntax for php 5.3
  undo change on Select.php, and leave the test case
  Fixes #5162

Conflicts:
	tests/ZendTest/Db/Sql/SelectTest.php
Closes #5855
Merge branch 'hotfix/5855' into develop

* hotfix/5855:
  fix T_OBJECT_OPERATOR on php 5.3
  fix array syntax for php 5.3
  undo change on Select.php, and leave the test case
  Fixes #5162
Zend\Db\Adapter MySQLI driver CS fix, don't unset properties
Merge pull request #5943 from mtymek/hotfix/console_matcher_test

Fixed route matcher test
Merge branch 'hotfix/5943' into develop

Close #5943
Merge pull request #4021 from ralphschindler/hotfix/db-mysqli-connection-unset-to-null

Mysqli driver raise a lot of warning about Undefined property after connection closed
Merge branch 'hotfix/4021' into develop

Close #4021
[#5942] skip some tests when ext/intl is not present

- When @ralphschindler ran using a PHP version that did not have
  ext/intl, these two tests failed. They should not run in that
  situation.
Merge #5942
Merge branch 'hotfix/mvc-translator-bc' of git://github.com/weierophinney/zf2 into hotfix/5942

* 'hotfix/mvc-translator-bc' of git://github.com/weierophinney/zf2:
  [#5942] skip some tests when ext/intl is not present
  Use --prefer-source flag for composer
  [#5942] Ensure TranslatorServiceFactory remains BC with < 2.3.0
Merging #5942
Merge branch 'hotfix/5942' into develop

* hotfix/5942:
  [#5942] skip some tests when ext/intl is not present
  Use --prefer-source flag for composer
  [#5942] Ensure TranslatorServiceFactory remains BC with < 2.3.0
Fix console route matcher was lowercasing optional value params' names.
Fix console route matcher was lowercasing optional value params' names.
Merge pull request #5939 from Xerkus/hotfix/mail-getheaderbyname-annotations

Fix annotation on Zend\Mail\Message::getHeaderByName
Merge branch 'hotfix/5939' into develop

Close #5939
Partially addresses #5845
Merge pull request #5951 from Thinkscape/hotfix/console-mixed-case-optional-value-params

Fix console mixed case optional value params
Merge branch 'hotfix/5951' into develop

Close #5951
Merge branch 'develop'

Promote develop to master, becoming the next stable release.
[2.3.0] Updated VERSION, README, CHANGELOG

- Updated `Zend\Version\Version::VERSION` to 2.3.0 (stable)
- Updated README with release date, non-dev label
- Added 2.3.0 changes to CHANGELOG
Merge branch 'releases/2.3.0'

2.3.0 readiness