Fix string shown for ./installed file if writing it fails.

The version was hardcoded previously. Now the entire string (version, date) is set in a variable which is used both for the installation script's try at writing the file and then for echo'ing to the user if the automated writing fails.

Note that I haven't actually tested this, so it may break in all sorts of ways. Also, please let me know if I broke coding style with something. This is my first patch to this project after all. :)