fix "Error while reading config: ErrorException: A non-numeric value …

…encountered in /www/xxxx/Parser.php:450 "

The exception is not thrown by yaml itself but by PHP because this line in Parser.captureSegment():
    result += _result;

Gets naively compiled in PHP (result is a Dynamic).
So I propose to change it to:
    result += Std.string(_result);

Stack trace:
    #0 /www/xxx/lib/yaml/Parser.php(450): php\Boot::php\{closure}()
    mikestead#1 /www/xxx/lib/yaml/Parser.php(2284): yaml\Parser->captureSegment()
    mikestead#2 /www/xxx/lib/yaml/Parser.php(574): yaml\Parser->readPlainScalar()
    mikestead#3 /www/xxx/lib/yaml/Parser.php(1169): yaml\Parser->composeNode()
    mikestead#4 /www/xxx/lib/yaml/Parser.php(553): yaml\Parser->readBlockMapping()
    mikestead#5 /www/xxx/lib/yaml/Parser.php(1757): yaml\Parser->composeNode()
    mikestead#6 /www/xxx/lib/yaml/Parser.php(991): yaml\Parser->readDocument()
    mikestead#7 /www/xxx/lib/yaml/Parser.php(812): yaml\Parser->parseAll()
    mikestead#8 /www/xxx/lib/yaml/Yaml.php(33): yaml\Parser->parse()
    mikestead#9 /www/xxx/lib/yaml/Yaml.php(49): yaml\Yaml::parse()

src/yaml/Parser.hx

@@ -307,7 +307,7 @@ class Parser
 				}
 			}
 
-			result += _result;
+			result += Std.string(_result);
 		}
 	}