Postgis fix for Trusty

[cotsog]

(Disclosure: Postgis is out of my comfort zone)

I was wondering if there's something we could do to support Postgis out of the box on Trusty. I think it was the case at some point but now customers need to manually install the following APT package: postgresql-9.5-postgis-2.3.

Here's the public issue about this: https://github.com/travis-ci/travis-ci/issues/6972. We've also had some reports in HelpScout.

Thank you for having a look!