Purge PostgreSQL 9.1 and 9.2

They make testing more problematic.
There's likely a better way but we don't support 9.1 and 9.2 anyway

.travis.yml

@@ -1,7 +1,7 @@
 before_install:
   - sudo apt-get install -q postgresql-server-dev-9.3
   - sudo apt-get install -q postgresql-9.3-postgis-2.1
-  - dpkg -l | grep postgresql
+  - sudo apt-get purge postgresql-9.1 postgresql-9.2
   - # Install schema_triggers
   - hg clone https://bitbucket.org/malloclabs/pg_schema_triggers &&
     cd pg_schema_triggers && make && sudo make install && cd -