Install specific postgres versions

This is meant to solve the issue with postgis and the error message
"GEOS 3.4 or higher required" as that error is compiled statically into
postgis. That basically means the build was using a wrong postgis
binary.

.travis.yml

@@ -25,7 +25,11 @@ before_install:
 
   # Install postgres db and build deps
   - sudo /etc/init.d/postgresql stop # stop travis default instance
-  - sudo apt-get -y install postgresql-9.5 postgresql-server-dev-9.5 postgresql-plpython-9.5 postgresql-9.5-postgis-2.2 postgresql-9.5-postgis-scripts
+  - sudo apt-get -y install postgresql-9.5=9.5.2-2ubuntu1
+  - sudo apt-get -y install postgresql-server-dev-9.5=9.5.2-2ubuntu1
+  - sudo apt-get -y install postgresql-plpython-9.5=9.5.2-2ubuntu1
+  - sudo apt-get -y install postgresql-9.5-postgis-2.2=2.2.2.0-cdb2
+  - sudo apt-get -y install postgresql-9.5-postgis-scripts=2.2.2.0-cdb2
 
   # configure it to accept local connections from postgres
   - echo -e "# TYPE  DATABASE        USER            ADDRESS                 METHOD \nlocal   all             postgres                                trust\nlocal   all             all                                     trust\nhost    all             all             127.0.0.1/32            trust" \