installer: Abstract out version of postgres installed.
installer: Abstract out version of postgres installed.

This allows for installing from-scratch with a different pinned
version of PostgreSQL, and provides a single place to change when the
default should increase.

showell/zulip,brainwane/zulip,zulip/zulip,punchagan/zulip,eeshangarg/zulip,eeshangarg/zulip,hackerkid/zulip,zulip/zulip,brainwane/zulip,brainwane/zulip,brainwane/zulip,andersk/zulip,andersk/zulip,rht/zulip,rht/zulip,synicalsyntax/zulip,rht/zulip,punchagan/zulip,zulip/zulip,timabbott/zulip,timabbott/zulip,zulip/zulip,kou/zulip,punchagan/zulip,showell/zulip,timabbott/zulip,showell/zulip,brainwane/zulip,zulip/zulip,synicalsyntax/zulip,zulip/zulip,hackerkid/zulip,brainwane/zulip,timabbott/zulip,andersk/zulip,synicalsyntax/zulip,rht/zulip,punchagan/zulip,synicalsyntax/zulip,kou/zulip,kou/zulip,hackerkid/zulip,hackerkid/zulip,showell/zulip,timabbott/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,hackerkid/zulip,andersk/zulip,brainwane/zulip,punchagan/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,rht/zulip,hackerkid/zulip,punchagan/zulip,showell/zulip,synicalsyntax/zulip,synicalsyntax/zulip,kou/zulip,showell/zulip,andersk/zulip,zulip/zulip,hackerkid/zulip,timabbott/zulip,andersk/zulip,rht/zulip,synicalsyntax/zulip,kou/zulip,timabbott/zulip,showell/zulip,punchagan/zulip,eeshangarg/zulip,eeshangarg/zulip,rht/zulip