Test against Postgres 9.5 as well as 9.4

[richvdh]

Postgres 9.5 is the first to support UPSERTs, so we should really run against
it as well as 9.4.

[codecov-io]

Codecov Report

Merging #4676 into develop will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #4676      +/-   ##
===========================================
+ Coverage    75.17%   75.19%   +0.01%     
===========================================
  Files          340      340              
  Lines        34712    34712              
  Branches      5677     5677              
===========================================
+ Hits         26096    26101       +5     
+ Misses        7013     7008       -5     
  Partials      1603     1603

[erikjohnston]

I would be sorely tempted to only test pg9.4 on py2.7 and a later postgres on 3.6, but meh.

Do we want to test py3.7 instead of 3.6?

[richvdh]

I would be sorely tempted to only test pg9.4 on py2.7 and a later postgres on 3.6, but meh.

maybe... we can always change it later I guess.

Do we want to test py3.7 instead of 3.6?

probably, but it's slightly non-trivial. Will make it a separate PR.