Add support for fetching specific versions of releases.

[dark5un]

• While adding this feature we realised that we could not run the tests because building with alpine:edge resulted with an error. So we included this fix as well.

This also addresses #14

cc @crsimmons

[vito]

Hmm I'd rather not switch out the base image as part of this. Would you mind removing that from this PR? It looks like it's failing on the openssl package, which I also saw happening in another resource. I can just remove it from the list.

Also jfyi concourse/concourse#1707 will probably be the way to express this kind of thing in the future. I'll merge this for now though since we can always address it if/when we make this resource space-aware.

[crsimmons]

@vito Thanks.

I've removed the base image change. Looking forward to spatial resources when they land.

[vito]

thanks!