Bump version number (pre-release 0.17.9)

Adding `unGrab` to X.Operations is potentially a user-config breaking
change (Ambiguous occurrence ‘unGrab’ …), so we need xmonad-contrib to
conditionally reexport this (instead of just deprecating the now
duplicated definition). Thus we need a version bump here.

Related: 0156e29 ("X.Operations: Add unGrab")

xmonad.cabal

@@ -1,5 +1,5 @@
 name:               xmonad
-version:            0.17.2.9
+version:            0.17.9
 synopsis:           A tiling window manager
 description:        xmonad is a tiling window manager for X. Windows are arranged
                     automatically to tile the screen without gaps or overlap, maximising