[ci] use PostgreSQL 9.4 on Travis

By default, Travis uses 9.2. Rational for upgrading:
* 9.2 is EOL
* Rails tests against 9.4 (since before 5.0.0.beta2, 9.3 before that)
* Some tests break on 9.2

Author: dr-itz

.travis.yml

@@ -42,6 +42,8 @@ rvm:
   - jruby-9.1.16.0
 jdk:
   - openjdk8
+addons:
+  postgresql: "9.4"
 env:
   - DB=mysql2 PREPARED_STATEMENTS=false
   - DB=mysql2 PREPARED_STATEMENTS=true