Less trademark-encumbered doc icons (fixed #1787)

Thanks to http://p.yusukekamiyamane.com/
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.