Add 9.6 to postgresql alternate versions
Add 9.6 to postgresql alternate versions

and bump the default from 9.1 => 9.3

spurti-chopra/travis-cookbooks,ljharb/travis-cookbooks,travis-ci/travis-cookbooks,spurti-chopra/travis-cookbooks,travis-ci/travis-cookbooks,ljharb/travis-cookbooks,travis-ci/travis-cookbooks,ljharb/travis-cookbooks,spurti-chopra/travis-cookbooks