How to install Electrum wallet in an offline Linux computer?
I needed a special command to use local storage, which at last worked:

sudo pip install Electrum-2.7.12.tar.gz --no-index --find-links `pwd`


UPDATE: Whenever I've had to do this for the second time, even the first instructions (posted in my question), to get the apt-get dependencies recursively, didn't work. So finally I decided to create a script that does everything for you: https://github.com/knocte/offlineElectrum/blob/master/downloadElectrumAndDeps.fsx