Use the correct variable as the key into $record

It was using $fieldName, which is the CSV field name, not the database
field name. This prevents duplicate detection from working. It now
properly uses $SQL_fieldName
Merge pull request #1821 from oddnoc/csvbulkloader-findexisting-2.4

Use the correct variable as the key into $record
composer/installer dep workaround

Moved to end of requirements, to work around a bug in composer - see https://github.com/composer/composer/issues/1147. This caused the dependencies to be installed in the wrong folder because the custom 'silverstripe-module' instructions hadn't been loaded at the time the core modules were installed via composer.
Fixing array to string conversion in dev/build for PHP 5.4
Updated github path
Merge remote-tracking branch 'origin/2.4' into 3.0

Conflicts:
	README.md
	composer.json
	docs/en/installation/from-source.md
	docs/en/misc/contributing.md
	model/Database.php