revisions to installation instructions

- Remove references to PostgreSQL 9.0; 9.1 is recommended
  for Evergreen 2.4.
- Clarify that the latest version of OpenSRF now means 2.2
  or later.
- Increase chances that somebody following the instructions
  precisely doesn't run into a problem during the configure step.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Mike Rylander <mrylander@gmail.com>
update upgrade instructions for 2.4.0 release

- specify minimum supported PostgreSQL version as 9.1
- specify minimum supported OpenSRF version as 2.2.0
- update schema upgrade instructions and miscellaneous
  references to 2.3

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Mike Rylander <mrylander@gmail.com>
adjust release notes references to refer to 2.4.0

Signed-off-by: Galen Charlton <gmc@esilibrary.com>
Signed-off-by: Mike Rylander <mrylander@gmail.com>