Updated CHANGELOG.md for version 1.3.1 release
Updated CHANGELOG.md for version 1.3.1 release

frankmayer/ArangoDB-PHP,frankmayer/ArangoDB-PHP,arangodb/arangodb-php,diabl0/arangodb-php,arangodb/arangodb-php,diabl0/arangodb-php,cesarsmaniotto/arangodb-php,cesarsmaniotto/arangodb-php