Bump version to 2.1.9

- Substitute occurrences of "2.1.8" with "2.1.9" using:
  git grep -I -l -e '2\.1\.8' | xargs \
  sed -b -i -e 's/2\.1\.8/2\.1\.9/g'

CUETools.Processor/CUESheet.cs

@@ -24,7 +24,7 @@ namespace CUETools.Processor
     {
         #region Fields
 
-        public readonly static string CUEToolsVersion = "2.1.8";
+        public readonly static string CUEToolsVersion = "2.1.9";
 
         private bool _stop, _pause;
         private List<CUELine> _attributes;