More version tags

[kichooo]

It would be nice to be able to use specific version of postgres:

for instance:

postgres:
image: healthcheck/postgres:9.5

[KaelBaldwin]

same for mysql

[tianon]

See my comment on #11 (comment) (referencing docker-library/cassandra#76 (comment)), especiall this bit:

... this repository exists precisely as an example for users to set up their own HEALTHCHECKs which are relevant to their deployments. Many users are going to need modifications, etc, which is why I'd honestly not recommend users use these images directly, and why I'd be hesitant to do things like tag multiple versions which might encourage more users to do so.