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/frmCUETools.cs

@@ -609,7 +609,7 @@ namespace JDP
         {
             get
             {
-                return "CUETools 2.1.8";
+                return "CUETools 2.1.9";
             }
         }