Updated format version written to files when using "Latest"

Though there aren't any relevant changes to the format in 1.11, apart
from support for embedded images on tilesets and image layers.

src/libtiled/fileformat.cpp

@@ -68,12 +68,13 @@ QString FileFormat::versionString()
         return QStringLiteral("1.8");
     case Tiled_1_9:
         return QStringLiteral("1.9");
+    case Tiled_1_10:
+        return QStringLiteral("1.10");
     case UnknownVersion:
-    case Tiled_Current:
     case Tiled_Latest:
         break;
     }
-    return QStringLiteral("1.10");
+    return QStringLiteral("1.11");
 }
 
 QString FileFormat::classPropertyNameForObject()