django-redshift-backend depends directly on psycopg2. #149

[shimizukawa]

for #149.

Previously, django-redshift-backend would let the user choose whether to use psycopg2 or psycopg2-binary. django-redshift-backend relies on psycopg2, so either one is required. However, if they were unaware of the option, they could create an environment where psycopg2 was not installed. So I changed the method to change psycopg2 to a direct dependency, and if you want to select psycopg2-binary additionally, install it with pip install django-redshift-backend[psycopg2-binary].