Test against Postgres 9.1 as well

Author: sfackler

.travis.yml

@@ -3,6 +3,8 @@ addons:
   postgresql: 9.4
 script:
 - cargo test
+- (sudo service postgresql stop && sudo service postgresql start 9.1)
+- cargo test
 - cargo doc --no-deps
 after_success:
 - test $TRAVIS_PULL_REQUEST == "false" && test $TRAVIS_BRANCH == "master" && ./.travis/update_docs.sh