Switch CI from Travis CI to GitHub Actions (#12)

Switch CI from Travis CI to GitHub Actions (#12)

Enable GitHub Actions and remove Travis CI for regression tests.

This commit enables GitHub Actions to run regression tests for pg_bigm.
GitHub Actions now tests on PostgreSQL 9.2 or later. Note that pg_bigm
supports PostgreSQL 9.1, but it can't be compiled on the latest Ubuntu
used in GitHub Actions, so tests are omitted for v9.1.

Also this commit discontinues the use of Travis CI as our CI service for
running regression tests due to issues that prevented successful
test execution.

Back-patch to pg_bigm 1.2.