Use pg_virtualenv for test.sh

Ref: https://github.com/yourlabs/django-autocomplete-light/pull/215#issuecomment-30764977
tests: use Django 1.6.1
tests: use `-r requirements.txt` for base requirements
tests: fix DJANGO_VERSION map for 1.6
Merge pull request #224 from blueyed/tests-pg_virtualenv

Fixes for test.sh: use pg_virtualenv