Check if we have write access to the target folder of wifi-completion.ba...

...sh before installing it. This comes in handy in case we are installing this wifi module into a virtualenv, in which in most cases we don't want to use superuser permissions.

I've to be honest: I don't know if this is the proper way of achieving my goal... so I've no problem if you reject this PR ;-), I just needed a clean installation via pip into a virtualenv for me.
Under other circumstances, I do think that we should follow that idiom, but in this case, we aren't writing to the test file, so it's sort of a weird proxy test to see if we have permission to write to that directory.

If you don't want to change it, I'll accept the pull request anyway, but it's just a little weird, no?