xdg_cache_dir() uses wrong environment variable

[Andrew Starr-Bochicchio]

config.py contains this:

def xdg_cache_dir():
    # See http://standards.freedesktop.org/basedir-spec/latest/ar01s03.html
    # Possibly this should be different on Windows?
    e = os.environ.get('XDG_CACHE_DIR', None)
    if e:
        return e
    else:
        return os.path.expanduser('~/.cache')

But according to the specification the environment variable name should
be XDG_CACHE_HOME, not XDG_CACHE_DIR.

Forwarded from Debian: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=736448