Postgis datasource tests

[springmeyer]

We are missing coverage for postgis. We need at least basic coverage to add regression tests when bugs are fixed or add tests to ensure changes don't regress key postgis plugin behavior in mapnik.

Plan:

• Add test/postgis.test.js and test/data/test_data.sql (copied from https://github.com/mapnik/test-data/blob/master/sql/table1.sql)
• test/postgis.test.js will bootstrap loading a test database. We will expect that the system already has a postgis enabled template_postgis database, that we will use as a template. If a template_postgis does not exist, the tests will be skipped
• Install postgis via mason package on travis, ensure tests are not skipped on travis by writing a separate script to ensure the test database is created.

[springmeyer]

getting done in #809 809

[springmeyer]

Closing, will track at #809.