MATE version detection/get_wallpaper is broken with MATE >= 1.10

[James Lu]

Using Variety 0.6.0 / Ubuntu 16.04.

The problematic comparison is here: https://bazaar.launchpad.net/~peterlevi/variety/trunk/view/head:/data/scripts/get_wallpaper#L29

Version strings of "1.12.0", "1.10.0", etc. are not greater than "1.6.0" when doing string comparison, so newer versions of MATE cause Variety to fall back to mateconftool-2. This in turn makes get_wallpaper fail with a "command not found" error.

Some solutions would be to replace the simple string comparison with more in-depth version checking (though granted, there isn't an extremely clean way of doing this: https://stackoverflow.com/questions/4023830/), or to simply drop support for MATE versions < 1.6.

More specifically, I get errors like this in the console:
/home/james/.config/variety/scripts/get_wallpaper: line 32: mateconftool-2: command not found
ERROR: 2017-03-17 22:11:42,803: get_desktop_wallpaper() 'Exception when calling get_wallpaper script'
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/variety/VarietyWindow.py", line 2301, in get_desktop_wallpaper
    output = subprocess.check_output(script).strip()
  File "/usr/lib/python2.7/subprocess.py", line 574, in check_output
    raise CalledProcessError(retcode, cmd, output=output)
CalledProcessError: Command '/home/james/.config/variety/scripts/get_wallpaper' returned non-zero exit status 127