Fix build_venv.sh script.

Proper usage of shell tests.
Also print which Python binary and python version is being used.
Dropping 2.6 from travis testing since it fails.

Originally added on the off chance that Python 2.6 actually worked.
Don't depend on venv when running make test.

Since it is now set up so dependencies can be installed into an
alternate virtual environment (or even the system environment) don't
assume that venv has to be built before running tests.
Updates to README and INSTALL regarding docs and venv.