TravisCI builds all failing with postgresql errors during capybara integration tests

We have a Rails 3.2.21 application that we've had under continuous testing via TravisCI for many months now. Recently we merged a large feature branch to master and since then every build has been failing with Postgres related errors:

ActiveRecord::StatementInvalid: PG::ConnectionBad: PQconsumeInput() server closed the connection unexpectedly

ActiveRecord::StatementInvalid: PG::ConnectionBad: PQsocket() can't get socket descriptor:

It seems to fail out on a different test each time, so doesn't appear to be related to a specific tests. These tests all pass when run locally on Mac OS X environments.

We've tried rolling back to Postgres v9.3 from v9.4, though these same tests passed under 9.4 both on Travis and locally. We've tried setting connection pool really high (20), really low (1) and not at all.

We did add a new database exclusion constraint which is the only database related change we think might be relevant, the other changes are pretty standard table creations, column renaming or moving, etc.