clean-old-theme.sh is executed properly when running install.sh from another directory

[SidRoberts]

Previously install.sh would fail to run clean-old-theme.sh unless launched from the source directory:

~ $ cd ~
~ $ /tmp/Qogir-theme-master/install.sh
'gnome-shell' not found, using styles for last gnome-shell version available.
/tmp/Qogir-theme-master/install.sh: line 722: ./clean-old-theme.sh: No such file or directory
Installing '/home/sidroberts/.themes/Qogir'...
Installing '/home/sidroberts/.themes/Qogir-Light'...
Installing '/home/sidroberts/.themes/Qogir-Dark'...

Done.