Docs: Changes to README to reflect PG 9.4 minimum version

Docs: Changes to README to reflect PG 9.4 minimum version

Include some notes about our new use for the PostgreSQL community's
apt source for Debian Wheezy and Ubuntu Trusty makefiles.

Clean up text about PG versions that come with supported Linux
distros for install.

State PG 9.4 as recommended version, with PG 9.3 as minimum

Note: may need to add note about future deprecation of PG 9.3
support in next releases of Evergreen

Signed-off-by: Ben Shum <ben@evergreener.net>