Travis: Use PostgreSQL 9.3.

The persistent test is failing and I have no idea why, as it's not
displaying any error message.  This commit changes a few things with
PostgreSQL just to see if they make any difference.

Note that the persistent test suite passes with flying colors both
before and after this commit on my machine using PostgreSQL 9.4.2.

.travis.yml

@@ -7,6 +7,9 @@
 services:
  - redis-server
 
+addons:
+ postgresql: "9.3"
+
 # The following enables several GHC versions to be tested; often it's enough to test only against the last release in a major GHC version. Feel free to omit lines listings versions you don't need/want testing for.
 env:
  #- CABALVER=1.16 GHCVER=6.12.3

serversession-backend-persistent/tests/Main.hs

@@ -19,7 +19,7 @@ P.mkMigrate "migrateAll" (serverSessionDefs (Proxy :: Proxy SessionMap))
 
 main :: IO ()
 main = hspec $
-  forM_ [ ("PostgreSQL", createPostgresqlPool "host=localhost user=test dbname=test password=test" 20)
+  forM_ [ ("PostgreSQL", createPostgresqlPool "user=test dbname=test password=test" 20)
         , ("SQLite",     createSqlitePool "test.db" 1) ] $
     \(rdbms, createPool) ->
   describe ("SqlStorage on " ++ rdbms) $ do