Work-around for travis trusty postgis problem

See https://github.com/travis-ci/travis-ci/issues/6972

.travis.yml

@@ -1,4 +1,5 @@
 language: c
+dist: trusty
 
 before_install:
   - sudo apt-get update
@@ -15,6 +16,9 @@ install:
 
 addons:
   postgresql: "9.3" # for "installcheck"
+  apt:
+    packages:
+      - postgresql-9.3-postgis-2.3
 
 # Note: Valgrind currently reports many problems when libght is enabled. So for
 # now, and until the problems are fixed, we just run the unit tests with libght