Testing PostgreSQL extensions on Travis CI revisited

My previous attempt to setup up multiple-PostgreSQL-version testing on Travis CI worked OK, but didn't actually make good use of the features of Travis CI. So I stole, er, adapted an idea from clkao/plv8js, which uses an environment variable matrix to control which version to use. This makes things much easier to manage and actually fires off parallel builds, so it's also faster. I've added this to all my repositories for PostgreSQL extensions now. (See some examples: pglibuuid, plxslt, pgvihash, pgpcre, plsh)