remove unsupported postgres versions

[asherf]

Remove support for version on that are no longer supported by the postgre team

It is worth mentioning that the docker image for postgres 9.1 fails to build due an issue w/ apt-get and debian indexes.
see https://travis-ci.org/asherf/postgres_exporter/builds/546034279

[coveralls]

Coverage remained the same at 61.844% when pulling 8b7b49f on asherf:remove-old-postgres into 734dc50 on wrouesnel:master.

[asherf]

master is essentially broken right now (see #275 which just changes a readme).
this seems to fix it.
@wrouesnel I think merging this will release a bunch of PRs open for this repo that are currently blocked because of test failures. I am happy to help rebase those and push PRs for them once this is in.

[wrouesnel]

Thanks!