update ownCloud if necessary

this will always download the latest ownCloud and upgrade if ownCloud install dir exist, this apphroach allows us to keep existing user plugins. currently not checking if currently installed version is equal to the one we're downloading as I couldn't find a proper solution for that
fixes comments by @JoshData
this is what I meant
install/upgrade ownCloud 7.0.2

Merge branch 'master' of github.com:jkaberg/mailinabox