Better testing coverage for travis

This improves the testing coverage by PostgreSQL to include optional PostGIS and plperl tests, in addition to testing MBD_SCRATCH_PG* environment variables.

I also had to tweak the PostGIS and plperl tests to work with PostgreSQL 9.3

I will send you at least two more PRs as I complete them:

 1. Tests need to work when MBD_SCRATCH_ is defined but no postgres server binaries
 2. Documentation for MBD_SCRATCH

So please wait for them before doing a release.