It turns out that some popular libaries still have problems with OpenSSL
1.1, particularly `postgres` 0.15. So we'll build two sets of images
now.
We also explain the new image tagging scheme, and fix a bug wher the
test script didn't test the newly built image.
Fixes#27, or at least works around it.
This deals with the issues mentioned in
https://github.com/sgrif/pq-sys/pull/18, and it relies heavily on ideas
from @golddranks and @clux.
