Travis Postgres server update to 9.4

Once the CI officially supports 9.5, we can switch to that. Switching to
9.5 will verify us that the atomic upsert feature of postgres works as
expected.

.travis.yml

@@ -33,7 +33,7 @@ matrix:
     - mongodb
   - env: GHCVER=7.6.3 CABALVER=1.18 BACKEND=postgresql
     addons:
-      postgresql: "9.3"
+      postgresql: "9.4"
       apt:
         sources:
         - hvr-ghc
@@ -81,7 +81,7 @@ matrix:
     - mongodb
   - env: GHCVER=7.8.4 CABALVER=1.18 BACKEND=postgresql
     addons:
-      postgresql: "9.3"
+      postgresql: "9.4"
       apt:
         sources:
         - hvr-ghc
@@ -129,7 +129,7 @@ matrix:
     - mongodb
   - env: GHCVER=7.10.2 CABALVER=1.22 BACKEND=postgresql
     addons:
-      postgresql: "9.3"
+      postgresql: "9.4"
       apt:
         sources:
         - hvr-ghc