venv activation in Makefile

[zeileis]

Reto @retostauffer, in the review by @dmreagan in #24 it turned out that L31 of the "venv" target in the Makefile does not run, at least on some systems:

source venv/bin/activate && make install

This tries to first activate the virtual environment and then run the "install" target. Following a discussion on StackOverflow the alternative would be to use

. venv/bin/activate && make install

which worked on my system. However, given that other places of the documentation advise the users to activate the virtual environment themselves after running the "venv" target, I thought it would be cleaner to just remove this line.

I've changed the installation documentation correspondingly and make sure that the virtual environment is activated before running the "install" and "test" targets. This works on my system.

Let me know if you want to resolve this in a different way.

Final note: It seems that the "cov" target assumes that the virtual environment is activated somehow. I did not test this myself, but this might need adaptation.