simply unload EasyBuild module rather than fiddling with the environment

@geimer: please test
This will be insufficient if the EasyBuilld version being used to install EasyBuild with was installed with e.g. `easy_install --user`. Is there something better we can do?

The bootstrap script needs to be retested with this updated easyblock as well, so it has to be enhanced to allow passing it `easybuild-*` source tarballs rather than only pulling from PyPi.